A stone tool found in Syria more than 80 years ago has sharpened scientists’ understanding of Stone Age networking.
Small enough to fit in the palm of an adult’s hand, this chipped piece of obsidian dates to between 41,000 and 32,000 years ago, say archaeologists Ellery Frahm and Thomas Hauck. It was fashioned out of volcanic rock from outcrops in central Turkey, a minimum of 700 kilometers from where the artifact was found, the researchers report in the June Journal of Archaeological Science: Reports. Until now, the earliest transport of obsidian into the Middle East was thought to have occurred between 14,500 and 11,500 years ago, when Natufian foragers began to live in year-round settlements (SN: 9/25/10, p. 14). Someone probably shaped the obsidian chunk into a usable tool near its Turkish source, say Frahm, of Yale University, and Hauck, of the University of Cologne in Germany. The tool, which could have been used for various cutting and scraping tasks, was then passed from one mobile group to another, perhaps several times, before reaching Syria’s Yabroud II rock-shelter. Along the way, the implement underwent reshaping and resharpening.
The most direct path between the Turkish and Syrian sites stretches about 700 kilometers. But hunter-gatherers meander, following prey animals and searching for other food. So, Stone Age bearers of the obsidian tool probably traveled considerably farther to reach one of several rock-shelters clustered near what’s now the Syrian town of Yabroud, the investigators say. “They didn’t type ‘Yabroud’ into a GPS unit and make their way to the rock-shelter as fast as possible,” Frahm says. Excavations at the Yabroud sites between 1930 and 1933 yielded the obsidian tool and hundreds of artifacts made from a type of rock called chert found a mere five to 10 kilometers away. Some researchers suspect the obsidian tool was mistakenly included among much older finds shortly after being excavated. But a copy of the lead excavator’s book describing his fieldwork, housed at Yale, confirms that the implement was found in sediment dating to around the time ancient humans and Neandertals inhabited the Middle East, Frahm says. Since excavators did not collect material for radiocarbon dating, Frahm and Hauck estimated the Syrian rock-shelter’s age by comparing its sediment layers and artifacts with those at several nearby, better-dated sites. Neandertals survived in the Middle East and elsewhere until at least 40,000 years ago (SN: 9/20/14, p. 11), so they might have been the final recipients of the obsidian tool. But Frahm considers Homo sapiens a better candidate. Humans occupied the Middle East and nearby regions throughout the period when the tool may have been used. No hominid fossils have been recovered at the Syrian site.
Using a portable X-ray device, Frahm and Hauck determined the chemical composition of the obsidian tool and 230 obsidian samples from known sites throughout southwestern Asia. That let the researchers match the Syrian find to its Turkish source.
Outside the Middle East, previous evidence suggested that long-distance obsidian transport occurred in Stone Age Eurasia. Researchers reported in 1966 that two obsidian pieces with sharpened edges found at northern Iraq’s Shanidar Cave originated roughly 450 kilometers to the north. That analysis used an earlier technique for measuring a stone’s chemical composition. Shanidar’s obsidian finds date to about the same time as that of the Yabroud II obsidian tool, perhaps to as early as 48,000 years ago, Frahm says.
Recent investigations of obsidian artifacts at late Stone Age sites in Eurasia not far from Shanidar Cave, in what’s now Armenia and Georgia, indicate that hunter-gatherers there also exploited vast territories, says archaeologist Daniel Adler of the University of Connecticut in Storrs. Frahm has contributed to some of that research. As for the Yabroud II obsidian tool, “a 700-kilometer transport distance is fully within the realm of possibility for a single person over an extended period of time,” Adler says.
Eurasia may have a far older tradition of extensive hunter-gatherer networking than the Middle East does. Evidence of long-distance obsidian transport in Armenia dates to as early as around 500,000 years ago, notes archaeologist Andrew Kandel of the University of Tübingen in Germany. That means Neandertals or other now-extinct hominid species first transported obsidian across hundreds of kilometers, he says.
People may think they act independently. But we catch social behaviors faster than colds. Whether or not we vote, try a new food or wear clear plastic pants will have something to do with whether other people are doing it. Unfortunately, it’s often hard to prove exactly how contagious a particular behavior is, or which behaviors will actually spread.
A new study shows that among runners using a fitness social network, logging miles is infectious — if the runner you’re comparing yourself to is slightly worse than you. The work shows a clever new way to determine if a behavior is socially contagious. But the results also confirm something about runners: We might be a little too competitive.
Glance at any group of kids with fidget spinners or teens wearing the same brand of shoes, and it’s easy to buy into the idea of social contagions. But it’s actually not so simple to distinguish between that kind of peer influence and other outside factors. says Johan Ugander, an applied mathematician at Stanford University. It’s easy for marketers to say they want something to go viral. But it’s a lot harder to be confident that the thing that’s spreading is really a social contagion and not a common factor like location or a group of people with common interests.
For one thing, birds of a feather really do flock together. People become friends because they have things in common. You might share an important article to Facebook, and find a friend has shared the same article 20 minutes later. But is that because you influenced your friend? Or because you have the same interests and read the same kind of articles, and you just happened to see it 20 minutes before they did?
Similarly, people tend to have friends who are near them geographically. So if two neighbors gain weight, Ugander notes, it could be “because the McDonald’s opened down the street, not because I gained weight then you gained weight.”
To get around this problem of homophily, Sinan Aral, a computational social scientist at MIT, decided to look at a single behavior — running. “Running is one of the most pervasive forms of this type of exercise,” he says. “I thought it would be most generalizable.”
Runners also like to log their miles and share them with their peers. Aral was able to get data from one of several social media platforms designed for jogging buffs. Aral can’t disclose the exact app or program he used in the study (and received some funding from), but there are a few social apps out there that fit the bill, including MapMyRun, RunKeeper and Strava. Aral ended up with a vast, anonymized dataset of more than 1.1 million runners who logged more than 359 million kilometers (223 million miles) over five years. To figure out if running is contagious, there would have to be a way to make some people run and other people stay home. “The ideal study to show [a causal effect] would be to go around with a cattle prod, prodding some people to exercise more,” but leaving others alone, Aral says. “But that’s not an experiment you can run.” (Obviously.)
So Aral and colleagues used the weather as their cattle prod. Good weather beckons a runner outdoors. Rain, snow, sleet or hail will keep a runner off the street. The researchers combed through data, noting what the weather was like on a given day and how far a runner went. They then compared that runner’s logs to their friends in other cities — places far enough away that the weather would be different. With the weather serving as the prod for some to put on their running shoes or keep others at home, the scientists got a good idea of who socially infected who with the running bug.
They found that for every additional kilometer run by someone’s distant friends, the runner would add another 0.3 km (0.18 mi). Timewise, a friend spending an additional 10 minutes running will inspire an extra three minutes in someone else, Aral and his group reported April 18 in Nature Communications.
But social contagion didn’t affect everyone the same way. The runners most influenced were those who compared themselves to runners slightly worse than they were. “In the loose analogy of a race, I’m more motivated by the guy behind me than I am motivated by the person ahead of me,” Aral says.
The study also showed some gender effects. Men were most influenced by other men, but were also ready to outrace the women. In contrast, women did not tend to be influenced by men. “There have been experimental studies showing men are more competitive with one another, while women are more self-directed and motivated by internal factors,” Aral says. But that doesn’t mean that women aren’t competitive. Women were just influenced by other women.
“It’s usually very difficult to identify causal effects from correlational data, in the absence of a randomized controlled trial,” says Edoardo Airoldi, a statistician at Harvard University. The data in this study may not translate to other behaviors — even to other forms of exercise. But that isn’t the point. The beauty, Airoldi says, is in the methods. “People should know you can use the weather as an instrument to uncover patterns of human behavior that are otherwise hard to elicit,” he says.
Ugander is also a fan of the new study, but he notes that this doesn’t mean all social behaviors, or even all sports, will prove similarly contagious, especially with regard to the gender differences. “It’s not clear that that will generalize to yoga or some other sport with other gender norms,” he says.
We don’t really know what exactly drives social contagion. One theory — the complex contagion theory — posits that costly behaviors (like going for an exhausting run) require a lot of signals from a lot of people. In other words, the more people in your network are running, the more likely you are to run.
Another theory developed by Ugander’s group posits that it’s not quantity that matters, but variety, a theory called structural diversity. Your family all going running is one thing. A family member, a friend and coworker is another. “If you’re getting it from two people from different parts of your life, say sibling and coworkers, it carries more weight,” Ugander says. “But two coworkers probably got it from each other.”
A third theory suggests that embeddedness — the number of mutual connections — matters most. After all, seeing all your friends virtuously running and knowing that they are also seeing each other live the wholesome life is a lot of peer pressure.
The data from Aral’s group suggests that for running, embeddedness and structural diversity may prove the most catching. “This doesn’t mean the complex theory is wrong,” Aral notes. “We just didn’t see it for running.”
Aral himself is a runner, and admits he’s not immune to the social contagion he found. “When I have friends who are close to me in fitness in terms of their level of activity, I may be influenced,” he says. “I also feel like I’d be more influenced by someone who was slightly less of a runner than I am. If they were running more, I would think, ‘Wow, even so-and-so is out there, I need to get out there.’” This social behavior might be catching, but the vulnerable population already has a raging competition infection.
The rift in Antarctica’s Larsen C ice shelf continues to rip. Researchers from Project MIDAS, which tracks the effects of a warming climate on the ice shelf, report that the crack grew 17 kilometers between May 25 and May 31.
The crack has now turned toward the water and is within 13 kilometers of the edge of the shelf. Within days, the crack could reach the edge. When that happens, one of the largest icebergs ever recorded will fall into the ocean.
“There appears to be very little to prevent the iceberg from breaking away completely,” the researchers write.
After calving such a massive section, the shelf won’t be stable. It may experience the same fate as Larsen B, which disintegrated in 2002, after a crack there broke off a huge chunk of ice.
A weird new particle imitator flouts the established rules of particle physics. The discovery could help scientists simulate how particles behaved just after the Big Bang or lead to the development of new devices with unusual electromagnetic properties.
The curious new phenomenon involves a particle-like entity called a quasiparticle, formed from a jostling mosh pit of electrons that collectively act like a single particle in a solid. Found in a compound of lanthanum, aluminum and germanium, the new quasiparticle is a bit of a renegade, physicist M. Zahid Hasan of Princeton University and colleagues report June 2 in Science Advances. Known as a type-II Weyl fermion, the quasiparticle breaks a rule called Lorentz symmetry, which states that the laws of physics are the same no matter the observer’s perspective, whether moving or stationary. Lorentz symmetry is the foundation of Einstein’s special theory of relativity, which details the physics of observers zipping along near the speed of light. For a real particle, violating Lorentz symmetry would be an unallowable faux pas, but for quasiparticles, the rules are looser, so type-II Weyl fermions can behave in a way a normal particle wouldn’t.
Fermions are a class of elementary particle that includes quarks, which make up protons and neutrons, and electrons. There are three different types of fermions: Dirac, Majorana and Weyl. Dirac fermions are the garden-variety type and include electrons and quarks. Majorana fermions are their own antiparticles. Neutrinos, notoriously lightweight and elusive particles, could be either Majorana or Dirac fermions; scientists aren’t yet sure which.
Weyl fermions are a massless variety of fermion. No examples have been found in particle physics. But the quasiparticle version of Weyl fermions burst onto the scene in 2015, when scientists first discovered them in a compound made of tantalum and arsenic (SN: 8/22/15, p. 11). Soon, scientists realized that their Lorentz-violating relatives, type-II Weyl fermions, might likewise pop up in solids.
In the new study, Hasan and colleagues measured the relationship between the energy and momentum of the quasiparticles, showing that they were consistent with type-II Weyl fermions. Although previous experiments have shown hints of the unusual quasiparticles, those measurements were skin-deep, assessing particles only on the surface of the material, Hasan says. With surface measurements alone, it’s hard to confirm the type-II Weyl fermions are there, says physicist Alexey Soluyanov of ETH Zurich. But Hasan and colleagues peered inside the material. “Experimentally, this work really is a nice example,” Soluyanov says.
In solids, Weyl fermions lead to unusual behavior. Put a normal material in a magnetic field, and resistance to the flow of electricity grows, but in a solid with Weyl fermions, a magnetic field makes current flow more easily. Type-II Weyl fermions are even stranger, due to their Lorentz-violating properties. In a material with these quasiparticles, a magnetic field in one direction can increase conductivity, while in another direction it can decrease conductivity. “This type of thing can have interesting applications,” says Hasan. “In a single material, just by changing the direction of the field, now we can get different behaviors,” flipping between insulating and conducting, for example. The new material could also provide insight for particle physicists. “Of course, people have wondered what happens when you break Lorentz invariance,” says physicist Adolfo Grushin of Institut Néel in Grenoble, France. Type-II Weyl fermions could help scientists better understand theories that violate the rule. “It’s a good test-bed,” says Grushin.
For example, Hasan says, “we can test theoretical ideas in the early universe,” simulating how particles may have behaved just after the Big Bang, when Lorentz symmetry may not have been obeyed.
In pumped-up sequels for scary beach movies, each predator is bigger than the last. Turns out that predators in real-world oceans may have upsized over time, too.
Attack holes in nearly 7,000 fossil shells suggest that drilling predators have outpaced their prey in evolving ever larger bodies and weapons, says paleontologist Adiël Klompmaker of the University of California, Berkeley. The ability to drill through a seashell lets predatory snails, octopuses, one-celled amoeba-like forams and other hungry beasts reach the soft meat despite prey armor. Millions of years later, CSI Paleontology can use these drill holes to test big evolutionary ideas about the power of predators. “Predators got bigger — three words!” is Klompmaker’s bullet point for the work. Over the last 450 million years or so, drill holes have grown in average size from 0.35 millimeters to 3.25 millimeters, Klompmaker and an international team report June 16 in Science. Larger holes generally mean larger attackers, the researchers say, after looking at 556 modern drillers and the size of their attack holes.
Prey changed over millennia, too, but there’s no evidence for a shift in body size. The ratio of drill-hole size to prey size became 67 times greater over time, the researchers conclude.
It’s “the rise of the bullies,” says coauthor Michal Kowalewski of the University of Florida in Gainesville.
All these data on shell holes allow researchers to test a key part of what’s called the escalation hypothesis. In 1987, Geerat Vermeij proposed a top-down view of evolutionary change, where predators, competitors and other enemies growing ever more powerful drive the biggest changes in their victims. This wasn’t so much an arms race between predators trading tit for tat with their prey as a long domination of underdogs repeatedly stomped by disproportionate menace. (Unless the prey somehow flips the relationship and can do deadly harm in return.) Vermeij, now at the University of California, Davis, and others have drawn on escalating threats to explain prey evolutionary innovations in thick shells, spines and spikes, mobility, burrowing lifestyles and toxins. One aspect of escalation scenarios has been especially hard to test: the idea that predators can become more dangerous and a stronger evolutionary force over time. Drill holes suggesting bigger, more powerful attackers allowed a rare way of exploring the idea, Klompmaker says. He now reads the deep history as showing predators escalated in size, but prey didn’t.
The energetics worked out, in large part, because early hard-shelled prey called brachiopods — a bit like clams but with one shell-half larger than the other — became scarcer over time, while clams and their fellow mollusks grew abundant. Mollusks typically have more flesh inside their shells than brachiopods, and prey overall grew denser on the ocean bottom. Killer drillers, able to dine at this buffet, could thus support bigger bodies even when prey size wasn’t rising, too.
Prey don’t make drilling easy, Klompmaker says. An hour’s work gets a typical modern predatory snail only about 0.01 to 0.02 millimeters deeper into a mollusk shell. So finally striking lunch could take days of effort with the thickest shells. And that’s with specialty equipment: A snail alternates grinding away using a hard, rasplike driller and then switching to its accessory boring organ that releases acids and enzymes, weakening the drilling spot for the next bout.
The role of such animal clashes in evolution has been notoriously difficult to study, says marine ecologist Nick Dulvy of Simon Fraser University in Burnaby, Canada. Nutrients, climate and other factors that don’t swim away into the blue are much easier to measure. Even after a robust century of ecological study, “the discoveries that otters propped up kelp forests, triggerfishes garden coral reefs, and wolves and cougars create lush diverse watersheds are comparatively recent,” Dulvy says. Until the new drill-hole study, he could think of only one earlier batch of evidence (crabs preying on mollusks) for the long rise of predators as an evolutionary force.
The story from drill holes, says Vermeij, is “very convincing.”
PORTLAND, ORE. — A great flower shape for a moth trying to get a drink in the dark turns out to be awful from the plant’s point of view.
Offering hawk moths (Manduca sexta) a range of 3-D printed flowers with different curvatures shows that a moderately curved trumpet shape lets moths sip most efficiently, Foen Peng reported June 24 at the Evolution 2017 meeting. That’s a win for a nocturnal flying insect searching for nectar. Yet drinking ease wasn’t best for the plant. During swift sips, the moths did less inadvertent bumping against the artificial flowers’ simulated sex organs than moths struggling to sip from an inconvenient shape. Less contact with real flower parts would mean less delivery and pickup of pollen.
Peng, of the University of Washington in Seattle, offered the moths three other shapes besides the gently curved trumpet. The best for the plant was a flat-topped “flower” with a right angle drop to a nectar well in the center. Previous work suggested that lack of curves made it very difficult for hawk moths hovering above a flower and extending their tonguelike proboscises to tap and probe the way to nectar in dim light.
Pollination at first glance may look like an easy mutualism evolving with the best interests of both plant and pollinator. But these experiments reveal a hidden, underlying conflict, Peng said.
BOSTON — Guidebook claims about the superior acoustics of the ancient Greek amphitheater of Epidaurus are a tad melodramatic. An actor’s voice can be heard in the back row, but whispers and other quiet noises cannot, acoustician Remy Wenmaekers reported June 28 at a meeting of the Acoustical Society of America.
The acoustics of the 14,000-seat theater, which dates to the fourth century B.C., are often touted as carrying faint sounds with extraordinary clarity. Wenmaekers and colleagues at Eindhoven University of Technology in the Netherlands positioned microphones at 264 spots throughout the theater and recorded a slow whooping sound projected from the stage that went from low to high frequency with time like a fire truck siren. The team also recorded sounds made by a voice simulator that mimics the frequency spectrum of a male speaker. These tests provided acoustic parameters such as sound strength and reverberation time for various spots in the audience. Then, in a lab, researchers determined the threshold for hearing noises such as a pin dropping or person whispering against the background noise of the theater.
With no roads of humming traffic nearby, the theater, which is still in use today, is remarkably quiet, especially when there’s no wind, Wenmaekers said. But sounds like tearing a sheet of paper or striking a match would be discernable only for someone sitting near the stage. The sound of a dropped coin would just barely be audible for someone seated in the back, but a dropped pin would be too quiet to hear. It’s still unknown just how far noise made by an audience member unwrapping a piece of candy carries.
The earliest asteroids were probably made of mud, not rock.
Radioactive heat in the early solar system could have melted globs of dust and ice before they had a chance to turn to rock, a new simulation published July 14 in Science Advances shows. The results could solve several puzzles about the composition of meteorites found on Earth and may explain why asteroids are different from comets.
Most knowledge about the first solid bodies in the solar system comes from meteorites called carbonaceous chondrites, thought to be chunks of the first asteroids. Their chemical compositions are almost identical to the sun’s — if you took all the hydrogen and helium out of the sun, you’d get the mineral ratios found in these bits of rock. That similarity suggests the first asteroids formed directly from the disk of gas and dust that preceded the planets. The composition also suggests that these rocks formed in the presence of water and at relatively low temperatures, around 150° Celsius.
It’s hard to explain all those features at once. If the original asteroids were bigger than about 20 kilometers across — and there’s no reason to think they weren’t — decaying radioactive elements inside them would have made the rock hotter than that. Some planetary scientists have suggested that the asteroids were porous, and water flowing through a primitive plumbing system cooled them. But the water should have stripped some elements from the rock, ruining their sunlike chemistry.
“It was a paradox,” says planetary scientist Philip Bland of Curtin University of Technology in Perth, Australia. Bland was modeling how those original globs of ice and dust could have compressed into solid rock, when it hit him: What if they weren’t rock at all? “At that moment, nothing has happened to force those grains together to turn it into a rock,” he says. That was just something everyone had assumed.
Bland reasoned that heat from radioactive decay would melt the ice, and the resulting body would be an enormous dollop of mud. The mud would suspend sediment particles, so they wouldn’t be stripped of their sunlike elements. And it would allow the early asteroids to be any size and remain cool.
Bland and Bryan Travis of the Planetary Science Institute, who is based in Los Alamos, N.M., ran computer models of how the mud balls would evolve. Convection currents, like those that move molten rock within the Earth’s mantle, would develop, helping to transfer heat into space, the models showed. After several million years, the ball would harden completely, yielding the asteroids seen today.
“It nails the paradox,” Bland says.
Mud balls could even explain the difference between asteroids and comets, he says. Comets, which are more icy than rocky and tend to live farther from the sun, may simply have formed later in the solar system’s history, when there was less radioactive heat available to melt them.
The model also showed that some asteroids would be muddy all the way through, while others would develop cores of larger grains, with a great mud ocean on top of them.
The latter result could describe not just asteroids but bodies like the dwarf planet Ceres, the largest object in the asteroid belt. Observations from NASA’s Dawn spacecraft showed that Ceres has a rocky core and may once have had an ocean that has since evaporated, says UCLA planetary scientist Edward Young. “That process may have been something like what they’re describing.”
Planetary scientist Brandon Johnson of Brown University in Providence, R.I., thinks the model will inspire more research. “I’m interested in it myself, actually,” he says. “It makes a lot of sense and paints a clear picture of what might have been happening.”
But Young is concerned that the model’s flexibility means it won’t make specific enough predictions for future work to test it. “It has so many knobs, you can get it to do whatever you want,” he says. “I’m trying to think of what the killer observation would be.”
Out-of-whack microbes in the vagina can raise HIV risk — and now there’s evidence that the makeup of the penis microbiome matters, too. The greater the number of anaerobic bacteria tucked under the foreskin, the more likely an uncircumcised man is to become infected with the virus, researchers report July 25 in mBio.
“This mirrors what’s been seen in women, but it’s the first study of its kind in men,” says Deborah Anderson, a microbiologist and gynecologist at Boston University School of Medicine. The data come from heterosexual Ugandan men followed for two years as part of a larger study on circumcision. Researchers swabbed the men’s penises to collect bacteria samples at the beginning of the two-year study. Then they compared the penile bacterial composition of the 46 uncircumcised men who became infected with HIV over the course of the study with that of 136 uncircumcised men who didn’t.
The total amount of penile bacteria didn’t differ, but men with higher levels of anaerobic bacteria were more likely to have contracted HIV, researchers found. Having 10 times more Prevotella, Dialister, Finegoldia and Peptoniphilus bacteria raised the risk of contracting HIV by 54 to 63 percent after controlling for other factors that might affect risk, such as condom use habits and number of sexual partners.
The results might help explain why circumcision cuts the risk of HIV, says Thomas Hope, a cell biologist at Northwestern University Feinberg School of Medicine in Chicago: Removing the flap of foreskin takes away a moist hideout for bacteria that thrive in oxygen-starved environments. But, Hope cautions, the study only draws an association between the microbiome and HIV — not necessarily a cause and effect.
It’s not clear how certain bacteria might raise HIV risk, but the new study revealed one possible clue: Men with more anaerobic penis bacteria also had higher levels of inflammatory cytokine proteins, which call immune cells to the scene.
“Specific bacteria might cause inflammatory response that would cause the immune cells to congregate in the penis, where they’re more likely to be exposed to the virus,” says study coauthor Cindy Liu, a pathologist at George Washington University in Washington, D.C. HIV targets particular immune cells, so recruiting an immune response to the penis might have an unintended consequence — a free ferry ride for the virus into the bloodstream. Liu and colleagues hope to test that explanation more thoroughly by looking at tissue samples from circumcised foreskins, and seeing whether there’s a relationship between the penis microbiome and the kinds of immune cells found in the foreskin.
Some of these same bacteria are also linked to increased HIV risk in women, and the microbes can be swapped between partners during sex. While practicing safe sex is still the best HIV-prevention strategy, topical creams that adjust the bacterial balance on the penis might someday help lower the risk of infection, Liu says.
A string of state-directed, targeted mass killings left a bloody stain on the 20th century. A genocide more recent than the Holocaust is providing new insights into why some people join in such atrocities.
Adolf Hitler’s many accomplices in his campaign to exterminate Jews throughout Europe have justifiably attracted the attention of historians and social scientists. But a 100-day spasm of unprecedented violence in 1994 that wiped out about three-quarters of the ethnic Tutsi population in the African nation of Rwanda has the potential to reveal much about how mass killings unfold at ground level. There is no guarantee that a better, although inevitably incomplete, understanding of why certain members of Rwanda’s majority Hutu population nearly eliminated a Tutsi minority will prevent future large-scale slaughters. The research is worth the effort, though, especially in a 21st century already marked by massacres of hundreds of thousands of people in western Sudan’s Darfur region and in Syria.
Researchers have an advantage in Rwanda. When hostilities ended, Rwanda’s government gathered extensive data on genocide victims and suspected perpetrators through a national survey. And local courts tried more than 1 million cases of alleged involvement in the violence, making the case documents available to researchers.
Genocide studies have often split offenders into organizers — mainly political and community leaders — and “ordinary men” who kill out of blind obedience to central or local authorities and hatred of those deemed enemies. But the extensive data from Rwanda tell a different story: An individual’s willingness to take part in genocidal violence depends on many personal and social factors that influence whether and how deeply a person participates, says sociologist and Rwanda genocide researcher Hollie Nyseth Brehm of Ohio State University in Columbus.
Nyseth Brehm’s findings may not apply to some of Rwanda’s most avid killers, who eluded capture and fled the country as soon as hostilities stopped. But when it comes to the ordinary citizens swept up in the deadly campaign, involvement was not primarily about following political leaders’ orders to eliminate Tutsis.
New reports by Nyseth Brehm and others fuel skepticism about the popular idea that regular folks tend to do as they’re told by authorities. And a fresh look at a famous 1960s psychology study adds further doubt that people will blindly follow orders to harm or kill others. In reality, only about 20 percent of Hutu men, an estimated 200,000, seriously injured or killed at least one person during the genocidal outbreak, estimates Rwanda genocide researcher Omar McDoom of the London School of Economics and Political Science.
“Why did four in five Hutu men not engage in the killing?” McDoom asks. That puzzle goes against the ordinary man thesis that “implies there are no individual differences in genocide participation,” he says. He suspects participation hinged on personal motivations, such as wanting to defend Rwanda from enemies or make off with a Tutsi neighbor’s possessions. Social circumstances, such as living in high-violence areas or having friends or family members who had already murdered Tutsis, probably played a role too. Nyseth Brehm agrees.
Local triggers Genocides often fester before exploding. In Rwanda, Tutsi rebels attacked the Hutu-led government and set off a civil war several years before mass killings started. A turning point came when unidentified forces killed Rwanda’s president, shooting down his plane on April 6, 1994. Over the next three months, the government orchestrated a massacre of Tutsis and any Hutus deemed friendly or helpful to Tutsis. Most scholars place the death toll at around 800,000, although estimates range from 500,000 to 1.2 million. Bands of Hutus scoured the countryside for their sworn enemies. Killings took place at roadblocks and in raids on churches, schools and other community facilities. Hutu women killed on a much smaller scale than men did, although they often aided those involved in the carnage.
In many parts of Rwanda, local authorities appointed by the national government recruited Hutu men into groups that burned and looted homes of their Tutsi neighbors, killing everyone they encountered, says political scientist Scott Straus of the University of Wisconsin–Madison. In his 2016 book Fundamentals of Genocide and Mass Atrocity Prevention, Straus describes how Rwandan recruitment efforts coalesced into a killing machine. Politicians, business people, soldiers and others encouraged Hutu farmers to kill an enemy described as “cockroaches” in need of extermination. Similarly, Nazis portrayed Jews as cockroaches and vermin.
Despite the Rwandan state’s best efforts to encourage nationwide Tutsi annihilation, local conditions shaped how the 1994 genocide unfolded, Nyseth Brehm reported in February in Criminology. She looked at 142 of the nation’s 145 municipalities, known as communes. Some experienced as few as 71 killings, while in others, as many as 54,700 people were murdered, she found.
Communes with the fewest killings were those that had the highest marriage and employment rates, Nyseth Brehm says. In those settings, mainly farming communities where people knew and trusted each other, most citizens valued a peaceful status quo and discouraged a descent into mass killing, she suspects. Curiously, violence was worse in areas with the largest numbers of educated people. That points to the effectiveness of anti-Tutsi teachings in Rwandan schools, Nyseth Brehm suggests.
Her study relied on data from a postgenocide survey, published in 2004 by Rwanda’s government, intended to document every person killed during the atrocity. Citizens throughout Rwanda told interviewers about individuals in their communities who had been killed during the outburst of slaughter. Reported and confirmed deaths were checked against records of human remains linked to the 1994 genocide. Comparisons were also made to Rwanda’s 1991 census.
However, any data on killings during mass violence, including from the Rwandan survey, will be incomplete, Nyseth Brehm cautions. So she also analyzed data from 1,068,192 genocide-related cases tried in local Rwandan courts from 2002 to 2012. Of particular note, although most nongenocidal murders in Rwanda are carried out by men in their 20s, the average age of accused genocide perpetrators was 34.7 years old, Nyseth Brehm reported in the November 2016 Criminology.
Hutu men in their 30s joined the genocidal fray as a way to fulfill adult duties by defending their communities against an outside threat, she suggests. Preliminary analyses show that perpetrators tended to cluster in families; if one of several brothers killed Tutsis, the others were far more likely to follow suit.
Additional scouring of court data indicated that Rwandans who had siblings convicted of genocide killings were especially likely to have murdered Tutsis themselves. In earlier interviews of 130 Rwandans, some who had killed Tutsis and others who hadn’t, McDoom similarly found that perpetrators tended to cluster in families.
Missing murderers Unfortunately, the Rwandan genocide’s most prolific players have eluded both the law and science, says political scientist Cyanne Loyle of Indiana University Bloomington. Investigators have so far interviewed only a handful of the powerful “big fish” who orchestrated the genocide, plus several hundred people tried and imprisoned for genocide participation. Survey and court data are limited to killers who either stayed in Rwanda after atrocities ended or were caught trying to flee the country.
But perpetrators with the most blood on their hands traveled in bands, wiping out tens of thousands of people at a time before hiding abroad, Loyle says. For instance, local officials lured large numbers of Tutsis to a school near the town of Murambi, where Hutu militias used machine guns, explosives and other weapons to kill more than 40,000 people in just three days.
“Scholars have studied Rwandans who killed on the sidelines while a larger and deadlier campaign was under way,” Loyle says. “They have mistaken a sideshow for the main event.”
Perpetrators of colossal atrocities at Murambi and elsewhere were less powerful than the government’s genocide masterminds, Loyle says. These “murderers in the middle,” however, were better equipped and far more effective at killing than common folk who got caught up in events, she contends.
There are no good estimates of how many members of large-scale killing squads escaped Rwanda and now live elsewhere. From 15,000 to 22,000 members of the Rwandan army and local militia groups were at large in the Democratic Republic of the Congo, near Rwanda’s border, in January 2003, according to a report by the International Crisis Group, a nonprofit organization.
Nyseth Brehm acknowledges the difficulty of accounting for genocide perpetrators who eluded justice. She and others, including Straus, have interviewed genocide offenders who stayed in Rwanda, often imprisoned for their crimes. Many of those who fled must have traveled in groups that murdered on a grand scale, she says. Those mass killers represent crucial missing data on who participates in genocide, and for what reasons. Vicious virtue In interviews by Nyseth Brehm, McDoom and others, perpetrators listed many reasons for joining the 1994 killing spree — hatred of Tutsis, a perceived need to protect nation and family, a desire to claim a neighbor’s property or a decision to join a suddenly popular cause, to name a few. Blind obedience to brutal leaders was far from the only reason cited.
That finding conflicts with the late psychologist Stanley Milgram’s interpretation of his famous “obedience to authority” experiments. Milgram described those trials, in which volunteers were told to administer increasingly intense shocks to another person, as a demonstration of people’s frequent willingness to follow heinous commands. He saw the experiments as approximating the more extreme situations in which Germans had participated in the Holocaust. On closer inspection, though, Milgram’s study aligns closely with what’s known about Rwandan genocide perpetrators, says S. Alexander Haslam, a psychologist at the University of Queensland in Australia. In Milgram’s experiments, as in Rwanda and Nazi Germany, “those willing to harm others were not so much passive ciphers as motivated instruments of a collective cause,” Haslam says. “They perceived themselves as acting virtuously and doing good things.”
Although Milgram’s tests upset some volunteers, most participants identified with his scientific mission to understand human behavior and wanted to prove themselves as worthy of the project, Haslam and psychologist Stephen Reicher of the University of St. Andrews in Fife, Scotland, conclude in a research review scheduled to appear in the 2017 Annual Review of Law and Social Science.
Milgram conducted 23 obedience experiments with New Haven, Conn., residents in 1961 and 1962 (SN: 9/21/13, p. 30). Most attention has focused on only one of those experiments. Volunteers designated as “teachers” were asked by an experimenter to continue upping the intensity of what they thought were electric shocks to a “learner” — who was actually in league with Milgram — who erred time and again on a word-recall test. Through screams, shouts and eventually dead silence from the learner, 26 of 40 volunteers, or 65 percent, administered shocks all the way to a maximum of 450 volts.
But experiments that undermined participants’ identification with the scientific mission lowered their willingness to deliver the harshest shocks, Haslam and Reicher say. Fewer volunteers shocked to the bitter end if, for instance, the study was conducted in an office building rather than a university laboratory or if the experimenter was not physically present. An analysis of data available from 21 of the 23 experiments finds that 43.6 percent of 740 volunteers shocked learners to the limit. Participants were most compliant when an experimenter encouraged them to continue shocking for the sake of the experiment (by saying, “The experiment requires that you continue”), the psychologists add. Participants never followed the order: “You have no choice, you must continue.”
Milgram’s archives at Yale University contain letters and survey responses from former participants reporting high levels of support for Milgram’s project and for science in general. Many former volunteers told Milgram that they administered shocks out of a duty to collaborate on what they viewed as important research, even if it caused them distress at the time. Still, Milgram’s recruits often admitted having had suspicions during the experiments that learners were not really being zapped.
Milgram was right that his experiments applied to real-world genocides, Haslam concludes, but erred in assuming that obedience to authority explained his results. From Milgram’s laboratory to Rwanda’s killing squads and Nazi concentration camps, orders to harm others are carried out by motivated followers, not passive conformists, he asserts.
If anything, that makes genocide all the more horrifying.
