Maybe you’re in line at the bank when it happens, or riding in a crowded elevator — viruses don’t care. Before you can fortify your body with gallons of orange juice, you’re pinned to your mattress with a hard plastic stick under your tongue. You were infected, quietly and with great force, by a tiny biological invader.

Transmission from host to host is the means by which pathogens like the flu, the common cold, warts, and more, replicate. Now a growing school of thought in the social sciences says infection isn’t confined to bacteria and viruses. “Social contagion” is an umbrella term for the variety of major life choices that seem to spread with the same level of viral force as the disease-carrying pathogens that can buffet our immune systems.

Social contagions burrow their way into our unconscious processes through friendly interactions, familial advice, and romantic persuasions. They compel us to get married now, get divorced now, and lose weight n– well, maybe next week.

We seldom know when we’ve been infected, because much of the time the decisions that define the course of our lives are made unconsciously. James Fowler, a political scientist at the University of California, San Diego, and the author of Connected: The Surprising Power of Our Social Networks and How They Shape Our Lives, argues that the various tiny, mostly unimportant choices we make day-to-day end up determining the path of our lives much more than our so-called big decisions.

“We’re making decisions all the time,” he told me. We choose our outfits, our breakfast, our choice of words at the water cooler. When we ultimately decide to take bigger leaps — think weight loss or a cross-country move — that choice isn’t up to us anymore. We may think we’re being present and rational, but the first domino may have actually fallen years ago. And that may have been the result of a social contagion.

My Life In Your Hands

In the world of social contagion research, two to five years is the agreed upon length of time it takes a contagion to infect its host. It’s just enough time to yield an effect, but also just long enough for us to forget why, exactly, we’re compelled to eat better or fork over a down payment on that house. We may have the final say, but months and months of incremental exposure have steered that decision for us. Consider the innumerable new fathers who step onto wobbly bathroom scales each day only to find they’ve gained a dozen pounds in the last year. Aghast, they blame their crumbling willpower, or a failed exercise regimen — failing to recall sleeve after sleeve of cookies they consumed alongside their pregnant wives. Social contagions are powerful because they are invisible.

Fowler is interested in why certain behaviors are so contagious and, importantly, what we can learn from them. To answer these questions, he and Nicholas A. Christakis, a Harvard physician and social scientist, turned to a well-known longitudinal study: the Framingham Heart Study. Since 1948, the study has produced some of the medical community’s deepest insights into cardiovascular health. In 1960, it discovered the link between cigarette smoking and heart disease. In 2002, it found obesity was a risk factor for heart failure.

But perhaps its greatest contribution to science is its stellar (and unprecedented) retention rate. The first batch of participants were recruited in 1948; Fowler and Christakis decided to look at the data from their children, who make up the second generation study, which began in 1971 and ran to 2003. From the initial 1971 pool of 5,214 people, by 2003 only 10 had dropped out. For Fowler, this was data-mining gold. “It’s truly incredible,” he laughs. “I can’t get my undergraduates to show up next week, and these guys are getting people to show up over the course of 30 years.”

After much analysis, the team found that obesity spreads through social networks in patterns. Your geographic distance from someone impacts your future weight gain less than your social distance from that person. In other words, you’re more likely to gain weight if a close friend you see only occasionally starts gaining weight than if your coworker in the neighboring cubicle is packing on the pounds. The data suggest this correlation is even stronger if you and your friend are the same gender.

Subway riders walk through the turnstiles while leaving the U.S. Open in New York September 4, 2007. REUTERS/Lucas Jackson

There are plenty of other social contagions. Divorce, for example, appears to spread through close social connections. Fowler and Christakis joined forces again in 2009 to revisit the Framingham data. They found that participants with an immediate friend who got divorced were 75 percent more likely to end their own relationships. One step removed — the friend of a friend — and the effect dropped to 33 percent. Any further removed and the effect was negligible.

Or consider childbirth: a miraculous occasion we may naïvely assume is very much a personal decision. Not true. Childbirth is as socially contagious as obesity and divorce, and perhaps even more so. A study released earlier this May found that women in their late 20s and early 30s were significantly more likely to have kids if their friends from high school had given birth within the last two years. Social scientist and lead author of the study, Nicoletta Balbo, says childbirth becomes attractive to couples in several ways. “If your friend starts having children, you might feel the pressure to conform to this parental tendency,” she said. Women may feel pushed to the fringes, that they have less in common with their friends and each additional stroller or diaper becomes yet another tick off the dreaded biological clock.

Succumbing to these influences can be pragmatic. Couples who have kids around the same time tend to use each other as learning experiences. They trade tips, horror stories, and (gently) used onesies. If a couple has doubts about whether they are ready to become parents, they can turn to their close friends who just had their first child, who, when they were deciding whether to conceive, turned to a couple they knew. The network grows and grows.

The Fated Salad

What emerges from this cascade of decision-making is a general notion of timing. It’s why we see dot-com and housing bubbles, baby booms and tipping points; when enough people of the same age collectively agree that certain priorities trump others, a generation is born. According to Jonah Berger, marketing professor at Wharton and author of Contagious: Why Things Catch On, all of it happens organically. “Following others acts as a shortcut,” Berger told me, but in a more profound sense, he said, “it also acts as information about what seems more correct to do.”

The problem is that, sometimes, the behaviors that seem socially “correct” end up being catastrophic. Many studies suggest that suicide and non-suicidal self-injury (NSSI) are transmitted socially, and mostly among young students. Last year, Canadian researchers showed that a schoolmate’s suicide made children ages 12 to 17 five times more likely to have suicidal thoughts. Similarly, obesity is both a social contagion and a bona fide public health epidemic. More than a third of the U.S. population is overweight or obese, and we’re catching it from each other.

Research also shows that teen pregnancy succumbs to the will of social forces. One 2011 study, for example, found teen girls were more likely to get pregnant if they had older sisters who also got pregnant in their teens. Data from the Department of Health and Human Services reveals teen mothers are more likely to drop out of high school, rely on public assistance (a yearly burden on taxpayers totaling upward of $28 billion), and raise children who suffer from health and behavioral problems.

On the other hand, the benefits of social contagions may outnumber their drawbacks. Consider the antidote to obesity: weight loss. People who try to lose weight on their own fail far more often than do two people who set the goal together. A study conducted in 1999 found people who dieted by themselves lost less weight than people who dieted with a friend or family member. Among the loner group, 76 percent completed treatment and 24 percent kept the weight off by the four-month mark. Social support, meanwhile, boosted rates to 95 percent completion and 66 percent maintenance.

“The more we’re aware of social influences, the more we can try to correct against them,” Berger said. This is why social contagions, despite their ability to infect, are not viruses. Your immune system’s response to viruses is automatic, but when it comes to choosing a sensible salad for dinner, you have control over that decision — despite the high probability that your dining partner will choose a juicy steak. So enjoy your salad. Just know your first forkful of lettuce was two years in the making.