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Spread the joy. A new study indicates that happiness can be transmitted from person to person.

Credit: Photographer's Choice/Getty Images

With a tanking economy and global violence on the rise, there's at least one thing to smile about: A pair of scientists is reporting that happiness can spread through social networks, meaning that friends of the cheery contract the happiness bug themselves.

The data behind the new findings come from, of all things, a massive study of cardiovascular disease. In 1948, researchers began collecting health and other information on U.S. adults as part of the Framingham Heart Study. Today, the project has data on more than 14,000 people, and it has helped researchers identify many of the major risk factors behind heart disease and stroke. Because the Framingham leaders, trying to track volunteers over many years, worried about losing contact with them, they asked all subjects to provide the name of a friend who would know how to find them if necessary. Often, those friends were also part of the study.

Nicholas Christakis, formerly a hospice physician at Harvard, and James Fowler, a political scientist at the University of California, San Diego, used these data to create a social network of nearly 5000 people. The duo then matched the information with various health data. Last year, they reported that weight loss and weight gain could "spread" through the network, meaning that a guy whose friends were overweight was more likely to pack on the pounds himself (ScienceNOW, 25 July 2007). Christakis and Fowler published a similar finding on smoking earlier this year.

Now, the two have turned to something more ethereal: mood. To measure this, they relied on a questionnaire that was part of a depression assessment in the Framingham study. The volunteers were asked to rate how strongly they agreed with four statements that described their mood in the previous week, including "I enjoyed life" or "I felt hopeful about the future." The 60% who gave the highest score on every question were deemed happy. The rest were considered unhappy, the two report online this week in the British Medical Journal. The happiness assessments came from three examinations that occurred between 1983 and 2001.

Christakis and Fowler found that when one person was directly connected to another--for example, a close friend or immediate family member--who was happy, the original individual was 15.3% more likely to be happy themselves. If the person was once removed--the friend of a next-door neighbor, perhaps--the increase was 9.5%. And an individual two times removed from the original person boosted their chance of happiness by 5.6%. Unhappiness could spread as well, though the chance of becoming unhappy if one's friend became unhappy was slightly lower than for happiness. "It's person-to-person-to-person transmission," says Christakis.

The effect only held for friends in very close proximity, however, a mile or less apart. The researchers also found that mutual friends had a stronger effect than when only one considered the other a buddy. As in their obesity work, Christakis and Fowler also found that same-sex friendships and neighbor relationships were more potent than those of the opposite sex.

The biggest difficulty in social-network studies like these is showing that effect is actually spreading. It's equally possible that happy people simply like spending time with other happy people, or that they share an environment of safe neighborhoods and good jobs that make them happy. Responding to the former argument, Christakis and Fowler say they found that happy and unhappy people still become friends, but that--over time--the mood of one evolves to match the other. As for environmental effects explaining the clustering, the researchers found that although next-door neighbors had an influence, those living farther down the block--where the environment was presumably no different--did not.

Some experts remain dubious. In an accompanying paper, for example, two economists argue that applying the same methods to qualities such as height and acne in teenagers indicates that these traits too are contagious, even though no one believes that growth spurts are caused by friendships with tall people. Christakis and Fowler also didn't control for certain things that might affect happiness in a community, such as crime, weather, or unemployment, says one of the economists, Jason Fletcher of Yale University.

"Their study would really not permit differentiation" among the different explanations for why happy people, or unhappy people, cluster together, Richard Rothenberg, an infectious disease specialist at Emory University in Atlanta who has studied how HIV spreads through social networks, notes in an e-mail message. That said, the work is enticing and "brings a smile to your lips, because it's a funny thing, that happiness should spread," says Rothenberg, but the proof is not quite there.

Christakis and Fowler are continuing to plumb their data, looking also at loneliness, depression, and alcoholism.

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Tools of the trade. Artist's drawing of a possible shaman burial, found near the Sea of Galilee.

Credit: P. Groszman (illustration drawn to scale)

Before there were priests or doctors, people seeking solace or treatment for an illness often called in a shaman, an intermediary between the human and spirit worlds. Archaeologists working in Israel now claim that a 12,000-year-old grave of a woman buried with various animal and human body parts is that of an early shaman. If true, it could mean that shamanism arose during a critical period in human cultural evolution.

Although largely supplanted by organized religion, shamanism is still widespread in Asia, Africa, Europe, and the Americas. For example, many Eskimo groups around the Arctic Circle practice shamanism. The roots of shamanism reach back at least to the ancient Greeks and possibly even to prehistoric times. Many archaeologists assume that shamanism preceded organized religion, and some see depictions of shamans in cave art from 15,000 years ago or earlier--although that interpretation is controversial.

But recent excavations at Hilazon Tachtit, a cave west of the Sea of Galilee in Israel, may provide new support for prehistoric shamanism. Hilazon Tachtit was occupied by the Natufians, a people who inhabited the Near East between about 15,000 and 11,500 years ago. Most archaeologists see Natufian culture as a transition between hunting and gathering and the sedentary lifestyles of early farmers. At Hilazon Tachtit, a team led by archaeologist Leore Grosman of the Hebrew University of Jerusalem has found the remains of at least 25 people, most in collective burials. But one was treated differently. A woman, about 45 years old when she died and whose pelvis and spine were deformed, was buried separately, accompanied by a menagerie of animal remains. Among her grave goods were tail bones from wild cattle, a wing bone from a golden eagle, the shells of 50 tortoises, and a large foot from another person.

The team, which reports its findings online today in the Proceedings of the National Academy of Sciences, notes that tortoises, cow tails, and eagle wings play a role in the ritualistic practices of many shamans today and that many societies ascribe special powers to physically disabled people. "It seems that the woman in the Natufian burial was perceived as being in a close relationship with these animal spirits," the authors write. They suggest that shamanism either sparked, or was the result of, the cultural upheavals that accompanied the agricultural revolution in the Near East.

"This is an extremely important report on a rare find at a critical time of cultural evolution," says Brian Hayden, an archaeologist at Simon Fraser University in Burnaby, Canada. Ian Kuijt, an anthropologist at the University of Notre Dame in South Bend, Indiana, adds that the "authors have done an excellent job of supporting their argument" for prehistoric shamanism. But Mina Evron, an archaeologist at the University of Haifa in Israel, cautions that there may be alternative explanations, though she doesn't offer one herself. Just because the team's "colorful interpretation" seems plausible, she says, "it ain't necessarily so."

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Prehistoric toothache. These ancient teeth from Japan show tooth decay, which is more common in fertile women.

Credit: Daniel H. Temple University/University of Missouri

The old wives’ tale that a woman loses one tooth for every child she delivers may, in fact, contain a grain of truth. A new study has found that women have had worse dental health than men ever since our ancestors became farmers about 10,000 years ago. That wasn't just because their diets and eating habits changed, as researchers previously believed, but because women who settled on farms were more fertile than nomadic hunter-gatherers. A boost in fertility meant farmers' wives were pregnant more often, which caused changes in their hormones and saliva secretion that rotted their teeth, according to a report in this month's issue of Current Anthropology.

Researchers have known since the 1980s that the invention of agriculture led to more tooth decay, particularly in women. Most researchers have attributed this to dietary and cultural changes that come from settling down. Both men and women began eating more starchy grains, such as corn and wheat, which contain sugars. Changes in the division of labor meant that women were preparing food more than men--and snacking more, because they had access to more food. "You increase carbohydrates and generally you increase the incidence of dental caries," says anthropologist Clark Spencer Larsen of Ohio State University in Columbus.

The shift to farming also set in motion other important biological changes, notes biological anthropologist John Lukacs of the University of Oregon in Eugene. Lukacs did a meta-analysis of studies of tooth decay in 147 collections of tens of thousands of teeth from prehistoric and living humans that lived around the world from 12,000 years ago to 800 years ago. He confirmed that women consistently had more cavities than men when they lived in agricultural societies. He also documented a rise in fertility among women, perhaps in part because they were less nomadic and didn't have to carry children from place to place. Women would have experienced three other factors that occur during pregnancy and increase the number of cavities in women: a boost in female sex hormones; a reduction in the flow rate of saliva and its antimicrobial properties; and an increase in cravings for high-energy, sweet foods.

This comprehensive view of women's oral health is "very smart," says dentist and geneticist Alexandre Vieira of the University of Pittsburgh in Pennsylvania. How much being pregnant contributes to cavities could be tested, says Larsen, by comparing the incidence of tooth decay in women and men in modern populations before and after boosts in fertility. Lukacs agrees: "Our new task is to partition the factors that cause caries--how much is caused by biology and how much by culture," he says. "It's not all or nothing--it's a mixture."

Selfless.
Marmosets appear to practice "true" altruism.

Credit: Judith Burkhart

When it comes to sharing, brains are overrated. A new experiment shows that the common marmoset, a relatively primitive monkey, is willing to give food to unrelated marmosets even when there's no chance of having the favor returned. Scientists had speculated that such charitable behavior stemmed from more advanced minds, but this new research suggests that other factors drove the evolution of altruism.

Food sharing is often observed in the animal kingdom, but it usually results from self-interest or coercion. One recent study, for example, found that chimps would gladly help others gain access to a room full of food but only if the others asked for it by banging on the door (ScienceNOW, 25 June). Rats also share food but only after having been the recipient of food charity earlier (ScienceNOW, 5 July). Although both kinds of animals showed signs of altruistic behavior, they lacked "true" altruism.

To see whether marmosets are more selfless, a team of researchers led by anthropologist Judith Burkhart of the University of Zurich in Switzerland placed two of the monkeys in adjacent cages. The "donor" marmoset could reach one of two trays on a platform outside its cage. On each tray sat two dishes--one with a tasty cricket, the other without. When the donor monkey pulled a tray close, one dish came to it, while the second slid within reach of the "recipient" monkey next door. The researchers found that when another monkey was present, the donor was more than 20% more likely to pull the tray containing food to its counterpart. The donor was never rewarded for its good deed and knew it couldn't score a cricket by pulling the tray, but that didn't matter. It seems the marmoset simply felt the urge to feed a stranger.

Burkhart believes the marmosets' "spontaneous concern about the welfare of others" evolved not because of brainpower but because it helped the species survive as a "cooperative" breeder. Unlike chimpanzees, marmosets enlist help in child rearing. It is not uncommon for grandparent, aunt, or uncle marmosets to be involved in raising offspring, notes Burkhart, a social structure that the monkey shares with humans. The team presents its findings online this week in the Proceedings of the National Academy of Sciences.

Charles Snowdon, a psychologist at the University of Wisconsin, Madison, is working on a similar experiment with cotton-top tamarins, another monkey that breeds cooperatively. He says he's "excited" about Burkhart's paper and that it confirms some of his predictions about altruistic behavior in cooperatively breeding primates. Although Snowdon cautions that chimps and marmosets have many more differences than mere breeding style, he says that this research "suggests that social structure and social organization might be more important for organizing altruistic behavior than brain size."

Related sites

• Watch marmosets in action at the National Primate Research Center, UW Madison
• Common marmoset fact sheet
• A primer on primate social structure

Mixed-up world.
A female topi attacks a male to get him to mate with her.

Credit: Jakob Bro-Jørgensen

Who says males are always the persistent sex? Female topi--a type of African antelope--become so intent on mating repeatedly with the most desirable partner that males sometimes have to fend off their aggressive advances to avoid running out of sperm, a researcher reports. The study is the first to suggest that sperm depletion causes such a role reversal in a mammal.

Topi are "lek" breeders. For a month and a half each year, males congregate at a mating arena, or lek, to compete for barren patches of about 30 square meters. The biggest, fittest males, known as lek males, command plots in the center of the arena, and females, which come into heat for just 1 day per year, seek them out. These prized bulls mate as many as 36 times in just 30 minutes. A female copulates with about four males during her visit to the arena, usually mating with each male multiple times. Although they prefer lek males, nearly 75% of females also mate with less hunky males.

The males keep this up for the entire rut, only occasionally nipping off for a bite to eat, says behavioral ecologist Jakob Bro-Jørgensen of the Institute of Zoology in London. Bro-Jørgensen studies topi in Kenya's Masai Mara National Reserve, and the sheer length of the rut interested him in the role that sperm depletion might play. In related species, he says, repeated mating can deplete sperm, meaning that with each additional ejaculation, the male is less likely to fertilize the female.

Bro-Jørgensen reasoned that female topi, particularly if they have already mated with a less fit male, should want to mate as many times as possible with desirable males to maximize their chances of conceiving high-quality offspring. Conversely, males, which encounter many different females during the rut, pay an "opportunity cost" if they mate mainly with one female because they might run low on sperm.

As Bro-Jørgensen reports online 29 November in Current Biology, lek males with two females on their territory at once tended to focus their attention on the female they'd mated with less. The other female often became aggressive toward the mating pair (see movie), attempting to shift the male's attention back to her. However, her aggression sometimes backfired. Seven percent of the time, the male counterattacked. An aggressive rebuff was particularly likely when she'd already mated with him several times. According to Bro-Jørgensen, it's often assumed that males are the pursuers because for them, mating is less costly. However, he says, "In topi, there is a reversal."

According to behavioral ecologist Brian Preston of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, the finding "challenges conventional wisdom that males should mate whenever the opportunity arises." However, says Preston, the work "rests on an untested assumption that males do become sperm depleted. This has been shown only rarely in natural systems."

Related site

• A movie of a male being charged by one female while mating with another

Decider in chief.
A young queen (center) surrounded by larger drones and smaller workers.

Credit: Zachary Huang/Michigan State University

Royalty has its privileges, even in the insect world. Queen honey bees can choose the sex of their offspring, a new study shows. Like a sharp stinger, that finding pokes a hole in the notion that queens are merely mindless egg layers and that worker bees have the final say on whether the queen lays eggs that give rise to males or females.

Every young queen goes on a mating flight and then stores the sperm she collects from multiple matings for the rest of her life, using it up bit by bit as she lays eggs. Males, called drones, emerge from unfertilized eggs, and females emerge from fertilized ones and become the workers. So if the queen adds sperm to an egg, it will produce a female; if she withholds sperm, the egg will produce a male. That would appear to give the queen control over the sex of her offspring. However, the dogma among entomologists is that workers control the type of eggs the queen lays. The workers build the cavities, known as cells, in which the queen will lay her eggs. A queen will lay an unfertilized egg in a particular cell only if the cell is big enough to accommodate a male larva, which is bigger than a female one. So by controlling how many cells they build of each size, the workers can limit how many male offspring the queen produces.

Despite these constraints, the queen can still tip the gender balance of the hive, report Katie Wharton and a team of entomologists at Michigan State University in East Lansing. To prove it, they confined queens inside their hives in specially built cages. Each cage was placed so that the queen could not reach the large cells where she could lay drone eggs but only the small cells where she could lay worker eggs. After 4 days, the cage was removed and the queen allowed to roam free in the hive, which had ample empty cells of both sizes. The queen then sought out the larger cells and, on average, laid nearly three times as many drone eggs as usual, apparently making up for the skewed hive gender ratio that resulted from her incarceration, the researchers report in the November/December issue of Behavioural Ecology. "The workers and the queen clearly share control of honey bee demographics," Wharton says. "It was like discovering a checks-and-balances government inside the hive."

The queen's ability to make "her own decisions" adds a new layer of complexity to life in the hive and raises questions about what stimuli the queen is responding to, says Lars Chittka, an entomologist at Queen Mary University in London. "Is she remembering how many eggs she has laid, can she sense how much sperm she has used, or is there some sort of chemosensory cue telling her how many drone larvae are in the cells?" Chittka says. "Following this new research, it's anybody's guess."

Related site

• USDA honey bee research