Count the holiday cards on your mantel: That’s how many people care about you. That’s the premise of a 2003 study of social network size carried out by evolutionary anthropologists Robin Hill of the University of Durham and Robin Dunbar of Oxford and published in the journal Human Nature, anyway. Aiming to test the hypothesis that there’s a cognitive limit on the number of meaningful, reciprocal relationships—or, “friendships”—humans can maintain, Dunbar and Hill gathered information on the Christmas cards sent and received by 43 British households, ending up with data on the trajectories of nearly 3,000 cards. Christmas cards are a good measure of social networks, according to the Dunbar and Hill, because “In Western societies…the exchange of Christmas cards represents the one time of year when individuals make an effort to contact all those individuals within their social network whose relationships they value.”

Having discovered a correlation between the size of the average social group and a part of the brain called the “neocortex” among various primates, Dunbar had posited 150 as the maximum number of friendships humans can maintain. There was some ethnographic and historical evidence to support this hypothesis—150 happens to be the average size of most military units, Neolithic farming villages and hunter-gatherer communities—but the Christmas card study was the first empirical test of what came to be known as “Dunbar’s number.” As they predicted, Dunbar and Hill found that the average number of Christmas cards received by each household hovered around 150. The mean number sent out ranged from 11 to 149, averaging out to about 68, though many cards were addressed to couples or families. About 21 percent of cards were sent to family members. Dunbar and Hill concluded:

Even in contemporary western societies, where individuals are operating egocentric networks within a virtually infinite array of social possibilities, social network size and differentiation reflect the sociocentric networks observed in traditional societies, suggesting that the cognitive constraints on network size may apply universally to all modern humans.

But is the traditional Christmas card becoming obsolete? “The increase in social media, and the current financial climate, means that fewer Christmas cards are sent,” said Hill. Not only are eCards faster and cheaper to send en masse, but the ease with which we can share infinite holiday photos on Facebook makes choosing just one seem unnecessary. Sales of holiday cards in the U.S. dropped from 1.8 billion in 2009 to 1.5 billion in 2011. “Nevertheless, we would expect the cards to be sent to core members of the network, with electronic greetings for less important relationships. You could still use Christmas cards in a study today, although you might expect slightly different patterns to our original study.”

Even if email has made communication easier, the size of our neocortex hasn’t changed. “It is an interesting question whether people send more email cards,” said Dunbar. “I would hazard a guess not: The number of friends people have on Facebook is not significantly greater than 150, and on the whole people send cards only to those they care about.”