Why We're Willing to Host Thanksgiving Dinner

As we head into the United States' biggest food day of the year, you might find yourself wondering what's so valuable about your friends and family that you're willing to spend money and time cooking for them all—especially when half of them don't even do the dishes. New insight into why we share has been found, not around an American dinner table, but in the social networks of Amazonian tribes, who, like many of us, willingly shar a most valuable thing: beer.

The Tsimané people of the Bolivian Amazon make their living through fishing, hunting, and farming, as do many other indigenous populations in the region. They also brew beer with manioc, which is commonly shared with friends and family in rituals most of us know as "having a good time." But considering that brewing your own beer requires a lot of time investment, especially when you have to grow and harvest all the material yourself, simply giving it away would get old after awhile.

According to research on the arXiv pre-print server, Tsimané who host a manioc beer event are usually repaid in kind—and generally pretty quickly. It's evidence that, while the social benefits of having people over are valid—they write that "in addition to the food value of the beer, conviviality and intoxication create a social venue for conversation, exchange and friendship"—reciprocity is key to the development of social networks.

Why we're willing to do things that benefit others without benefiting ourselves, even if it's something as simple as hosting a gathering, is an age-old question for evolutionary biologists. The concept of reciprocity—that we do nice things because we expect people will eventually pay us back—can explain much of generosity, even in interactions we know will only happen once. But that generosity means we, and many organisms, can be taken advantage of by cheaters. Which brings us back to the Amazon: Are people more willing to share when they know they'll be paid back? And do family members, who we'd presumably have a genetic incentive to keep happy and healthy, get more benefit of the doubt?

The American team of researchers, led by Paul L. Hooper of the Santa Fe Institute, collected its study data in 2007 from a village of 198 individuals, spread across 35 nuclear families. In order to develop a social network based on beer sharing, the authors write, "nuclear families constitute nodes, while the daily frequencies of hosting constitute the weight of directed edges that connect them." To find out if kinship played a role, the team estimated relations based on collected genealogical data.

Based on more than 50,000 interactions collected over 16 weeks, the researchers write that "with respect to kinship, the current results are unambiguous." People were more likely to host their family members than others, which shouldn't come as much surprise—you always welcome your weird uncle (it's always the uncle) to Thanksgiving dinner because hey, he's family, even if he's not bringing much to the table.

As for whether or not people host guest because they expect to be hosted in return, the results are mixed. There is evidence that, within individual interactions, people do pay each other back. If family A hosts family B, B was observed to then be far more likely to host A within a few days.

But the correlation between hosting and sharing wasn't perfect.  The team found that there was far higher variability in how often people hosted guests—some a lot, some rarely—than there was in the probability that someone would attend when invited. This suggests that there isn't perfect reciprocity in the Tsimané society, as some people simply host more than others.

As the authors note, there are other factors that could come into play.  For example, as beer can be traded for help around the farm or for other purposes, some reciprocity may not fall directly into the host-host relationship. Also, the relationships being built quite possibly have longer time scales than could be observed in 16 weeks.

Still, as we head into the holidays, it's interesting insight into why we are willing to go through the hassle in the first place. Naturally, we get our own enjoyment out of the social interactions, and having people over can be a good motivation for doing something special you wouldn't normally take the time to do, like roasting a ham, or brewing beer. But when you're arm deep in dishes (or a turkey, for that matter), it's also nice to remember that next time, someone else will do it.

@derektmead