Erika L. Christakis is a lecturer at the Yale Child Study Center. Nicholas A. Christakis is a professor of sociology and of medicine at Yale University and co-author of “Connected: The Surprising Power of Our Social Networks and How They Shape Our Lives.”

In seeking to find out what led to the shootings at the Washington Navy Yard, we must ask the right questions. What features of American society make us more prone to mass homicides? Many will focus on the availability of guns. Others will note that mass homicides are overwhelmingly committed by men, or comment on the role that untreated mental illness can play. We must also examine the role of social isolation: It can be a tripwire for violence and mask our ability as a society to see violence coming.

Americans have always had dueling commitments, both in our national mythology and in our everyday lives, to rugged individualism and to civic engagement. We want people to stand alone, but we also want to band together.

Unfortunately, our zest for independence can become something far different: toxic social isolation. And our society doesn’t do well at bringing vulnerable people at our margins back into the fold. It’s not just for their sake that we should do so, however, but for our own.

In a national sample of Americans that we surveyed annually in collaboration with the Gallup organization, respondents were asked, “Who do you spend free time with?’’ and “Who do you discuss important issues with?” The study, published last year, found that U.S. residents identify an average of 4.4 close social contacts. On average, people identified the following contacts: 2.2 friends, 0.76 spouses, 0.28 siblings, 0.44 co-workers and 0.3 neighbors. At one extreme, 5 percent of Americans had eight such people with whom they were socially intimate. But at the other end of the spectrum, 8 percent of Americans reported having no one with whom they could talk about personal matters or spend free time. (This percentage has stayed relatively constant over time, despite the advent of the Internet and the increase in online and mobile interactions, according to the Pew Internet & American Life Project.) This isolated minority often suffers in silence.

The same Gallup survey followed up on socially isolated and more integrated people. It found that socially isolated individuals were less likely to donate blood, money or clothing; support political candidates; or volunteer to help prepare for a major public emergency. In other words, they lacked the social-emotional connectedness that many Americans take for granted. Such individuals might also be more prone to violence.

University of Chicago psychologist John Cacioppo and his colleagues, using the national Health and Retirement Study and other data, have documented the heavy burden that social isolation and, in particular, loneliness place on individuals. Loneliness increases one’s risk of depression, and Cacioppo and his colleagues have noted that it affects physical outcomes such as blood pressure, cholesterol level and the risk of early death. When people feel socially isolated, they increase their vigilance for threats and feel more vulnerable. Sometimes they even become paranoid. There is some evidence, from a 2005 paper in the British Journal of Psychiatry, that social isolation is a risk factor for the development of schizophrenia.

Unfortunately, as seems to be the case with Aaron Alexis, a mentally ill person’s social isolation makes it hard to recognize, or act on, the danger signs of impending violence.

We can’t know what went on in Alexis’s mind. Not all people who are alone feel lonely, of course, and extremely few go on killing rampages. But it is worth paying attention because social isolation harms not just those directly affected but also the majority who are not isolated.

Research we have conducted with political scientist and geneticist James Fowlerfound that a person’s sense of loneliness depends on how those in his social network are feeling. You are about 52 percent more likely to be lonely if a person to whom you are directly connected is lonely. But there is an even more extraordinary pattern at the edge of social networks. On the periphery, people have fewer friends; this makes them lonely, and it paradoxically drives them to cut their few remaining ties. But before they do, they may “infect” their friends with the same feeling of loneliness, starting the cycle anew. These reinforcing effects mean that our social fabric can fray at the edges, where it is weakest, like a yarn that comes loose at the end of a sweater.

To combat loneliness in our society, we should aggressively target the people on the periphery with interventions to repair their social networks. Even better would be to prevent some of these individuals from ever becoming socially isolated. Society’s many options to reduce risks of social isolation include targeting not just veterans suffering from stress disorders but also broader groups, such as young people who feel alienated, or bullied or the unemployed who feel unwanted. By helping those at risk of social isolation, we can protect our social fabric from unraveling.