One of the things the Buddha stressed very strongly in his teachings was being careful who we choose to spend time with. This is because our values and our mental habits will tend to align themselves with the values and mental habits of others.

At his bluntest he said things like: “Should a seeker not find a companion who is better or equal, let him resolutely pursue a solitary course; there is no fellowship with the fool.” (Dhammapada 61).

He also praised association with friends who embody skillful qualities:

“I do not see even a single thing that so causes unarisen wholesome qualities to arise and arisen unwholesome qualities to decine as good friendship (kalyana mittata). For one with good friends, unarisen wholesome qualities arise and arisen wholesome qualities decline.” (AN I, VIII, 1)

A new study with college roommates gives support to these beliefs by showing that a particular style of thinking that makes people vulnerable to depression can actually “rub off” on others, increasing their symptoms of depression six months later.

The research, from psychological scientists Gerald Haeffel and Jennifer Hames of the University of Notre Dame, is published in Clinical Psychological Science, a journal of the Association for Psychological Science.

Studies show that people who respond negatively to stressful life events, interpreting the events as the result of factors they can’t change and as a reflection of their own deficiency, are more vulnerable to depression. This “cognitive vulnerability” is such a potent risk factor for depression that it can be used to predict which individuals are likely to experience a depressive episode in the future, even if they’ve never had a depressive episode before.

Individual differences in this cognitive vulnerability seem to solidify in early adolescence and remain stable throughout adulthood, but Haeffel and Hames predicted that it might still be malleable under certain circumstances.

The researchers hypothesized that cognitive vulnerability might be “contagious” during major life transitions, when our social environments are in flux. They tested their hypothesis using data from 103 randomly assigned roommate pairs, all of whom had just started college as freshmen.

Within one month of arriving on campus, the roommates completed an online questionnaire that included measures of cognitive vulnerability and depressive symptoms. They completed the same measures again 3 months and 6 months later; they also completed a measure of stressful life events at the two time points.

The results revealed that freshmen who were randomly assigned to a roommate with high levels of cognitive vulnerability were likely to “catch” their roommate’s cognitive style and develop higher levels of cognitive vulnerability; those assigned to roommates who had low initial levels of cognitive vulnerability experienced decreases in their own levels. The contagion effect was evident at both the 3-month and 6-month assessments.

Most importantly, changes in cognitive vulnerability affected risk for future depressive symptoms: Students who showed an increase in cognitive vulnerability in the first 3 months of college had nearly twice the level of depressive symptoms at 6 months than those who didn’t show such an increase.

The findings provide striking evidence for the contagion effect, confirming the researchers’ initial hypothesis.

Based on these findings, Haeffel and Hames suggest that the contagion effect might be harnessed to help treat symptoms of depression:

“Our findings suggest that it may be possible to use an individual’s social environment as part of the intervention process, either as a supplement to existing cognitive interventions or possibly as a stand-alone intervention,” they write. “Surrounding a person with others who exhibit an adaptive cognitive style should help to facilitate cognitive change in therapy.”

According to the researchers, the results of this study indicate that it may be time to reconsider how we think about cognitive vulnerability.

“Our study demonstrates that cognitive vulnerability has the potential to wax and wane over time depending on the social context,” say Haeffel and Hames. “This means that cognitive vulnerability should be thought of as plastic rather than immutable.”

Of course it would be terrible if this was taken to mean that people with depressive tendencies should be shunned. It would be far better if, as the researchers suggest, they “surround” themselves others with more resilient mental habits, and avoid others who are disposed to react badly to setbacks.