PNAS First Look Blog

Science journalists discuss a selection of new papers from PNAS

Oxytocin can increase lying for the benefit of the group

Oxytocin is a hormone often thought of as a “love drug,” linked as it is with feel-good emotions such as trust, empathy and generosity. Increasingly, however, scientists find that oxytocin has a dark side — for example, it can spur envy and gloating, and also promote ethnocentrism, potentially fueling xenophobia, prejudice and violence. Now researchers find it might enhance lying that serves one’s group. The findings are detailed in the Proceedings of the National Academy of Sciences.

Past research has shown that oxytocin helps foster social bonding in mammals. Social psychologists Shaul Shalvi at Ben-Gurion University of the Negev in Israel and Carsten de Dreu at the University of Amsterdam in the Netherlands reasoned that in order to protect and promote the wellbeing of their group, people might bend the truth and behave unethically, a tendency potentially linked with oxytocin.

In an experiment, participants were each randomly assigned to a three-person group. They all had the same task — they saw a coin on a computer screen and were asked to predict the heads-or-tails outcome of a toss. When predictions were correct, the group won a small amount of money; when they were incorrect, the group lost nothing. All earnings were shared equally among group members.

A key part of the experiment was that volunteers did not have to type in their predictions beforehand. Instead, they typed in what their predictions were after seeing the outcome of the coin toss reveal whether their predictions were correct. This means that if they made an incorrect prediction, they could lie and report they predicted correctly.

When the participants could benefit their group by lying, 30 volunteers who inhaled oxytocin lied to a greater extent and more quickly than 30 volunteers who only received a placebo. For instance, when volunteers are honest, there is only a roughly 1 percent chance that people will correctly predict nine or all of 10 coin tosses, but when people received oxytocin, 53 percent of such volunteers reported such extreme coin-tossing “success.”

In another experiment, when lying benefitted only the individual, another group of 30 volunteers who inhaled oxytocin did not lie any more than 30 others who received a placebo.

“For our group members, or our loved ones, we will do almost everything, including lie and deceive,” Shalvi says.

These findings suggest that oxytocin might influence people to act in the best interest of their group, even if the actions include dishonesty.

“Oxytocin has been found to increase trust and cooperation in humans — as such, commercial applications has been proposed advocating oxytocin spray as a potential ‘love hormone,’” Shalvi says. “Our results cast doubt whether such applications should always be encouraged.”

Future work could test whether dishonesty that serves a group could help people in that group bond, Shalvi notes. Research could also see whether some people are more likely to lie for their group than others. “Are those individuals the leaders of the group, or perhaps people who aspire to become the leaders?” Shalvi asks. “Do people lie more when their group stands to profit because they feel that the responsibility for such act is shared and they are less likely to be punished? Alternatively, do they expect to feel less guilt from engaging in such group-serving lies? Or do they even anticipate feeling guilty for being honest?”

Categories: Psychological and Cognitive Sciences | Leave a comment

Plants can grow human antibodies

Immunoglobulin M, or IgM for short, is the first class of antibodies to appear in response to exposure to antigen. Increasingly, scientists are interested in IgMs as therapeutics, but growing them in mammalian cells at commercial scale is very challenging. Now researchers find they can synthesize IgM in plants, according to findings detailed in the Proceedings of the National Academy of Sciences.

IgM is by far the physically largest antibody in the human circulatory system, and it ranks among the most complex of human proteins — for instance, they are each made up of 21 to 24 polypeptides. Complicating the matter further is how roughly 10 percent of each IgM’s weight is made up of complex sugar-containing structures known as N-glycans, and improper addition of these molecules could impair antibody function.

Molecular biologist Herta Steinkellner at the University of Natural Resources and Applied Life Sciences in Vienna and her colleagues noted that plants have correctly assembled mammalian proteins such as immunoglobulin Gs before. They sought to see whether plants could also correctly fold and assemble complex IgM.

The scientists introduced the genes to produce the cancer-fighting IgM PAT-SM6 in a close relative of tobacco known as Nicotiana benthamiana. The anti-tumor IgM accumulated in the leaves of the plants and assembled itself correctly.

“I was surprised that plants can make such complex human proteins like IgMs,” Steinkellner says.

Analysis revealed the N-glycans were attached onto the plant-derived IgMs in a pattern resembling those seen on human-derived IgMs. The plant-derived IgMs behaved virtually identically to human-derived IgMs when it came to binding onto human lung cancer cells on lab dishes.

The researchers suggest their work opens the door for commercial production of similarly complex proteins for pharmaceutical  use. They note that future research using their system can investigate the role glycosylation has on the activity of antibodies and other vital proteins.

Categories: Applied Biological Sciences | Leave a comment

In vitro engineering of muscle tissue

Engineered skeletal muscle — the muscle under a person’s voluntary control — could help treat muscle disease and injury. However, previous in vitro attempts to engineer skeletal muscle with all the properties of actual muscle have failed.

Now scientists have developed muscle tissue that mimics the highly organized structure of real skeletal muscle. In addition, this novel muscle includes a reserve of functional stem cells that helps the tissue heal itself, a first for engineered muscle. When transplanted into mice, this engineered tissue rapidly integrated with the host blood vessel network over two weeks and developed structural, functional and regenerative properties comparable to real muscle. The findings are detailed in the Proceedings of the National Academy of Sciences.

The engineered muscle bundles were grown from rat myogenic cells — the cells that fuse together to form muscle fibers — within silicon rubber molds. Just like natural muscle, these bundles contained densely packed, highly aligned, cross-striated muscle fibers that each contained multiple cell nuclei.

Over time, the contractile force-generating capacity of this engineered muscle surpassed that of newborn or neonatal rat muscle and was 10 to 100 times greater than previous attempts at engineering skeletal muscle.

“For me, the most surprising and exciting finding was that our muscle was able to maintain a functional population of muscle stem cells responsible for tissue regeneration and growth, known as satellite cells,” says researcher Nenad Bursac, a tissue engineer at Duke University in Durham, NC. “Within our engineered muscle, these satellite cells responded to toxin-induced injury by proliferating and repairing damaged muscle fibers as well as contributing to the growth of new muscle fibers over time. Another very exciting thing was to observe day after day in live animals how our implanted engineered muscle becomes invaded by host blood vessels and is stronger.”

The scientists noted that developing engineered muscle can also enable them to perform novel studies of muscle growth and repair in a dish. It could also help them model different diseases in vitro, study them, and perform screening for new drug and gene therapies, Bursac says.

The researchers suggest that differentiating muscle cells before implanting them enhanced how well the engineered muscle did in live animals. For instance, fully differentiating them before implantation led to muscle with improved structure, function, and vascularization — that is, infiltration with blood vessels.

Bursac cautions their engineered muscle in its current form “is not applicable for clinical use, due to its small size and the cell source used for its production — neonatal rats. However, we believe that our work still shows significant advance in the field of skeletal muscle tissue engineering and identifies cell types and techniques that will accelerate the development of clinically relevant muscle.”

The researchers are now addressing the issues of cell source and size of the engineered muscle. “We have already started to make functional muscle from human muscle stem cells,” Bursac says. “While this engineered human muscle shows many features of normal muscle, we still need to improve its strength — that is, force that it can produce — to achieve what we achieved with neonatal rat cells.”

To create larger muscle tissues, the researchers will need to develop ways to incorporate a bed of capillary blood vessels within the muscle to help feed a significantly larger number of cells. “Many groups are working on this topic, as it pertains to all engineered tissues and not only the muscle,” Bursac says. “Another important part of the future development is the question if the implanted muscle will be able to integrate into the host neuronal system, such that it can be voluntarily moved as most of our muscles in the body are. For this we will need to develop methods for efficient innervation of engineered muscle after implantation.”

Categories: Applied Biological Sciences | Leave a comment

The mechanics of breastfeeding

“How do infants extract milk during breast-feeding? We have resolved a century-long scientific controversy,” say David Elad and his team from Tel Aviv University in Israel in the opening of their new PNAS article.

Until now, researchers have debated what forces from a suckling baby actually draw milk out of a woman’s breast. Is it peristalsis–a mouthing akin to milking a cow’s udder–or is it suction, a negative pressure created by the baby’s mouth?

The answer, Elad and colleagues say, is suction. To come to this conclusion, the team used two approaches. The first was to gather ultrasound movies of nursing babies–and to quantitatively analyze the frames to get an objective understanding of the process.

“When a baby is breastfeeding, first of all he/she is latching on to the nipple and areola complex, which means the baby is sealing the mouth–the baby is extracting into his/her mouth the nipple and areola complex,” notes Elad. Once a baby has latched on, the complex is stretched twice as long as the free nipple, as previous studies showed.
The observations also showed that the front part of a baby’s tongue acts like a rigid plate, moving up and down with the jaw as the baby nurses. Meanwhile, the elongated nipple moves back and forth in the mouth; the team measured this distance. The up and down motion of the front of the tongue, “definitely does not represent a

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peristaltic wave of contraction,” the authors write. “That is to say, the whole nipple is periodically compressed against the hard palate.” Taken together, the findings point to suction.

These observations allowed the team to do the second part of their analysis: creating the first computer model of breastfeeding mechanics. “We showed what this sub-atmospheric pressure should be in order for the nipple to be two times longer in the mouth,” Elad says, and they showed how it fluctuated with the motion of the nipple. It oscillated between about -20 and -40 millimeters of mercury after the baby latched on.

Oscillating pressure is not necessary: a constant pressure would work fine to extract the milk, Elad notes. But the baby has to do more than just draw milk. While a baby nurses, it is also swallowing and breathing, so the changing pressure and the jaw motion is probably a part of this rhythm. “If you watch movies of a baby nursing,” he says, “it’s just moving the mandible and it’s so relaxed.” Some literature reports say that babies drinking from artificial nipples on bottles have lower oxygen saturation than breastfeeding babies, perhaps because it is harder to settle into this ideal rhythm while bottle-feeding, he says.

Elad can’t say whether this mechanism applies to other animals. His next steps are to apply the same methods to studying babies with breastfeeding problems to try to gain insights into the problems’ underlying cause.

Categories: Biophysics and Computational Biology | Leave a comment

Future antibiotics: Keep bacteria from sensing each other

Alone, a single cell of Pseudomona aeruginosa—the bacteria blamed for many hospital-acquired infections—can’t cause much damage to the human body. In fact, the bacteria won’t even produce virulence factors, the compounds that make it pathogenic to humans, if it doesn’t sense neighbors. But add a few thousand other cells of P. aeruginosa, and suddenly the bacteria aren’t lone warriors; they’re a team. When they sense the presence of unique signaling molecules produced by their allies, the cells start making those virulence factors, ramping up to cause an infection.

“Bacteria evolved multicellularity,” says Princeton biologist Bonnie Bassler. “They’re so small that the only way they get any bang for their buck is by coordinating activities with each other.”

In a new PNAS paper, Bassler and her colleagues report the first ever molecule that stops P. aeruginosa from quorum sensing, that ability for cells to detect their neighbors and coordinate behavior as a group. By blocking quorum sensing, the researchers found, they can decrease the virulence of P. aeruginosa and its ability to form films of bacteria on surfaces, such as those inside the body.

Bassler already knew that bacteria relied on quorum sensing to synchronize many activities, including the production of virulence factors and the formation of so-called biofilms. And she and other scientists knew how to turn on quorum sensing, by simply adding the bacteria’s own signaling molecules to a sample, making cells think more neighbors were present. But the only way to stop bacteria from sensing their neighbors was to mutate the receptors that bound those molecules, obliterating the whole system for good.

“It was useful that we could use mutants to study the effect of having no quorum sensing,” says Bassler. “But mutants are always off, you can’t turn the system back on.”

Recently, though, Bassler’s lab found a molecule called chlorolactone that blocked one of the quorum sensing receptors in Chromobacterium violaceum, a bacteria found in water and soil. She wondered whether the chlorolactone—or similar chemicals—would also work to stop quorum sensing in the more medically important P. aeruginosa. So her group created 30 possible molecules and began testing them in the bacteria. One of them, called meta-bromo-thiolactone (mBTL), bound to a quorum sensing receptor and stopped the bacteria—even when in a group—from initiating behaviors that are synchronized through quorum sensing.

“What was surprising was that shutting down this one receptor really shuts down the whole system,” says Bassler.

With the successful results in isolated bacterial cultures, Bassler’s team began testing whether the drug would work against bacteria that were actively infecting human cells. So next, they cultured the bacteria with human lung cells. Once again, adding mBTL decreased the virulence of the bacteria.

“Now we have the luxury of having both probes that turn on quorum sensing and probes that turn off quorum sensing,” says Bassler.

The chemical isn’t immediately useful as a drug in humans, she says, but the work helps shed new light on how quorum sensing works and what receptors are most important in the process. Next, her group aims to repeat the experiments in more complex scenarios—when the cells are arranged in different ways on surfaces and have flow forces acting against them—to better mimic the environment inside the human body. They also want to know what other molecules could act against quorum sensing, and whether mBTL is effective in other species of bacteria.

“All the antibiotics that we currently have work by killing the bacteria or slowing their growth,” says Bassler. “The big question is whether

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there’s another way to treat bacterial infections.” Blocking quorum sensing, she says, could be how antibiotics of the future work.

Categories: Microbiology | Leave a comment

A genetic connection to alcohol consumption

Identifying genes for alcoholism has been tricky due to its complex nature. Now scientists have found a gene in rats linked to increased alcohol consumption and preference that could one day be a potential target for therapies, report findings detailed in the Proceedings of the National Academy of Sciences.

Alcoholism is a moderately to highly heritable disease. Although investigations have discovered several regions in the genome linked with alcoholism, singling out genes linked with complex disorders such as alcoholism is a complicated task. For instance, such disorders may involve many different gene variants that each may contribute only modest effects. Many of these variants may be uncommon or rare, hampering attempts to identify them. Also, which variants a person with such a disorder has may differ greatly across individuals.

To help uncover the genetic roots of alcoholism, neurogeneticist David Goldman at the National Institute on Alcohol Abuse and Alcoholism in Bethesda, Md., and his colleagues experimented with rats bred either to prefer alcohol or to shun it. They sequenced the genomes of both these breeds of rats and discovered variants in four genes were apparently linked with alcohol preference, one of which deactivated the gene Grm2, which encodes metabotropic glutamate receptor 2 (mGluR2) and plays a critical role with regard to the neurotransmitter glutamate.

“Glutamate is intensively studied for its many roles including memory and learning and neuronal damage following brain trauma,” Goldman says.

The researchers discovered rats that genetically lacked mGluR2 showed higher levels of alcohol preference and consumption. They also saw that injecting normal mice with a compound that inhibited mGluR2 led to a significant although possibly short-lived rise in voluntary alcohol drinking.

Goldman noted that glutamate is linked with many functions in the brain. As such, “it is highly likely that this specific genetic source of variation in glutamate neurotransmission will be multiple in its effects,” Goldman says.

A number of drugs already target glutamate neurotransmission. “Understanding that this neurotransmitter is involved in alcoholism and subject to extreme genetic variation could lead to development and therapies,” Goldman says. He cautions however, that their finding “is likely to take years to translate into diagnosis and treatment.”

Categories: Genetics | Leave a comment

Printing living cells with “woodblocks”

The inkjet printer technology often seen in offices is now finding use in research that uses suspensions of living cells as their ink to explore ways to manufacture complex tissues and organs. However, inkjet-based cell printing leaves many cells damaged or dead. Now researchers have developed a technique akin to ancient Chinese woodblock printing wherein nearly all cells survive. The findings are detailed in the Proceedings of the National Academy of Sciences.

Bio-printing may not only find use in manufacturing tissues and organs, but also in laying down patterns of cells on surfaces for a wide variety of experiments, such as testing if a molecule could find use in medicine, or analyzing how cells function and communicate with other cells. Previous inkjet-based cell printing typically can leave up to half the printed cells dead, lacks precision when it comes to depositing cells on surfaces, and overall can be expensive and difficult to operate.

“We were sick of using inkjet printing and started to think for other approaches to prepare a cell pattern,” says biomedical researcher Lidong Qin at Houston Methodist Research Institute.

Qin recalled that one day he and his colleagues were inspired by rubber stamps children play with. The way these stamps print letters and other images works much like the ancient Chinese woodblocks that revolutionized the printing world more than 1,800 years ago.

The blocks in question are made of silicone. Cells flow down columns in the blocks. The way in which these columns are arranged is fully configurable during the creation of the blocks. There are hook-like traps within these columns that help control the rate at which cells get deposited onto surfaces. When the block is lifted away, the cells remain behind in precise formations.

“As long as cells are suspended well enough, clogging won’t happen,” Qin says. “The hook-shaped traps are not sharp at their edges; tearing won’t happen either.”

This approach produces 2-D arrays of cells in as little as a half-hour. It prints the cells as close together as 5 microns — in comparison, most animal cells are 10 to 30 microns wide. It can simultaneously handle multiple cell types, and results in close to 100 percent cell survival. The researchers have named this method Block-Cell-Printing, or BloC-Printing.

The scientists demonstrated they could print metastatic cancer cells in a grid and examine their growth in comparison with non-metastatic cells to characterize the metastatic potential of those cells. This could help diagnose the stage of a cancer.

The researchers also printed a grid of brain cells and got them to form connections with each other. These cell junctions might be useful in future studies of neuron communication and regeneration for understanding Alzheimer’s and other neurodegenerative diseases.

“We found printing neuron

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cells was extremely challenging — it took us more than three months to figure out the right conditions,” Qin says.

The scientists noted the materials to make each block cost only about $1, and that each block is reusable for hundreds of printings. The approach does not require sophistical equipment — after a block has been fabricated, a researcher only needs a syringe, a suspension of living cells, a Petri dish and a steady hand, Qin says. In comparison, inkjet-based cell printers can cost between $10,000 and $200,000.

Qin does note that inkjet-based cell printing remains faster. Also BloC-Printing cannot yet print multi-layer structures as inkjets can. Still, Qin notes it could still find use for many types of experiments involving drug screening, RNA interference, and other molecule-cell and cell-cell interaction studies.

Categories: Biochemistry | Engineering | Leave a comment

New forests change land temperatures

Image: sxc.hu

Image: sxc.hu

Thirty years ago China began launching ambitious reforestation projects, aiming to undo some of the massive environmental damage caused by years of clear cutting. Each year the country adds around 2 million hectares of new trees, with no sign of slowing. China plans to add 40 million hectares of planted land between 2005 and 2020.

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team of Chinese and French scientists have asked what afforestation, the process of adding new trees, means for land surface temperatures. They present their findings in PNAS Early Edition. In some areas, they find, afforestation leads to an overall cooling effect, a plus for combatting climate change. In other areas, particularly in dry regions, afforestation leads to a net warming effect.

“These results suggest it is necessary to carefully consider where to plant trees to achieve potential climatic benefits in future afforestation projects,” write Shu-Shi Peng and co-authors.

Generally forests absorb more sunlight than grasslands, which have a higher albedo or reflection coefficient. This tends to increase the amount of heat on the land. However this effect tends to be offset by evaporative cooling. The cooling leads to an average decrease of around 1.1 °C during the day. Forested areas tend to have warmer temperatures at night however, an effect which

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increases with latitude and dryness. Wetter regions, the authors suggest, are better places to plant new forests.

The study drew on NASA satellite measurements of land surface temperature to compare reforested areas to adjacent crop and grasslands. This is the first study of its kind and is relatively simple in its comparisons. Future studies, the authors hope, will examine “the full range of climatic effects of afforestation by combining energy fluxes from eddy flux towers, satellite observations, and land surface models coupled with climate models.”

Categories: Environmental Sciences | Tagged , , | Leave a comment

Skin’s immune memory lasts a lifetime

When a patch of skin becomes inflamed—due to an infection or irritation—immune cells flock to the site to fight off any pathogens. Now, scientists have discovered that cells lodged in the skin keep the memory of such an infection for years afterward. The findings, reported in a new PNAS Early Edition paper, could lead to new ways to prevent or treat infections and chronic skin conditions.

“Knowing what we know about these cells now, we can potentially develop more effective vaccines,” says immunologist Scott Mueller of the University of Melbourne, a senior author of the new paper.

The presence of so-called skin-resident memory T cells was discovered by scientists only within the past decade. Memory T cells are generated after an infection, and retain the molecular marker of a pathogen. If that pathogen—a virus or bacteria—is encountered again, the memory T cells recognize it, and can launch a faster and more effective fight against it. For years, researchers thought that memory T cells only circulated in the blood. In 2004, multiple teams of scientists detected the presence of memory T cells within the skin and other epithelia, such as the lining of the lungs and stomach.

“Their location within tissues made it

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very hard for us to find them and to get them out to study,” says Mueller. “So they were basically overlooked.”

When they learned of the presence of the memory T cells in the skin, Mueller’s team wondered what happened to the skin-resident T cells after an infection cleared: did they remain in place, dissipate throughout the surrounding tissue, or disappear? To answer that question, the scientists closely observed sites of herpes simplex virus infections on the skin of mice, using molecular markers and microscopy. For months after the herpes infection disappeared, memory T cells that retained the signature of the herpes virus remained in place where the infection had been. Despite the ability for the cells to move, they didn’t dissipate away from the site as the researchers had expected.

“The discovery we made was that not only do they persist for long periods, but they persist right at where they were formed, at that site of the previous infection,” Mueller says. He expects that the memory T cells, in fact, remain at the site for the life of an

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The findings bring up a host of new questions about the function of the skin-resident memory T cells. “I think it’s pretty easy to guess why it would evolve to generate them in one site—if you get an infection in one spot, you might get it there again,” Mueller says. “I think the more interesting question is why it hasn’t evolved to then spread them out all over the skin in case you get an infection elsewhere.”

Mueller’s team is continuing to look at the biology of the unique memory T cells, studying whether they play key roles in other tissues of the body and why the skin-resident cells have a different physical appearance than other memory T cells. He also wants to know whether the immune cells lodged inside the skin are, at least partially, responsible for the constant inflammation associated with skin diseases like psoriasis and dermatitis. “This opens up all sorts of new avenues for studying diseases,” he says.

Categories: Immunology | Leave a comment

The value of a good reputation — It pays to be a good sport

Animals waiting to be shorn. Image Henry Lyle

Animals waiting to be shorn. Image Henry Lyle

Instead of finishing last, nice guys actually come out on top. Earning a reputation as a cooperative, hard worker pays off in community support, which can in turn lead to healthier families, report anthropologists in a recent paper in Proceedings of the National Academy of Sciences.

For decades scientists have wondered how cooperation is maintained within a group. Judged by standard evolutionary and economic models, an individual has the

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most to gain by free riding–benefiting from the cooperation of others but giving nothing in return. But when free-riding spreads this behavior soon causes a cooperative system to unravel.

In the Nuñoa District of southeastern Peru there is a small collective of 24 Quechua households. This group of indigenous families jointly owns herds, gardens, irrigation canals and buildings. Life in the mountains is hard for these people. Their livelihood depends, in large part, upon group-owned resources. Maintaining the resources often requires significant time, energy and expertise.

Anthropologist Henry Lyle was surprised when he learned that in this system lazy workers, or “low contributors,” received the same benefits as the hard-working, “high contributors.” He expected low contributors would receive, for instance, fewer potatoes from the community garden. However, says Lyle, this wasn’t the case:

“At this point I knew that high contributors must be gaining some benefit to offset the costs of their hard work and make them more tolerant of free riding.

“Here was a small group of folks who put a lot more effort into maintaining the communally-owned resources,” he says. “They were always present for work on these resources and worked harder during these tasks. They had

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a sense of camaraderie that wasn’t as strong among other community members. Clearly, they took care of one another.”

Lyle and his colleague Eric Smith found that community members who contributed the most time, energy and risk into group projects gained valuable reputations as reliable, hard workers. They were known as the most generous, influential and respected people in the community. But the benefits of a good reputation may extend beyond the merely social. High contributors attracted large social support networks and this aid,

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in turn, is associated with healthier families. The researchers think this link is causal, though it cannot be conclusively proven.

“My guess is that agricultural support access is positively correlated with production,” Lyle says. “So, the idea is that those who have larger agricultural support networks can better feed their families and, as a result, have healthier families.”

Some critics may contend that sicker families simply have less time and energy to contribute to community projects. But the author’s present several lines of evidence that this isn’t so. Morbidity and time invested in group tasks were not correlated. And after controlling for morbidity in a model, they still find investment in group tasks can predict a person’s reputation.

Other societies maintain cooperation by punishing free-riders. In Nuñoa the free riders themselves get a free pass. The value of a good reputation is enough, in seems,

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