Does your housemate have a strange, unexplainable effect on your behavior ? Well, there's a gene for that…and that gene belongs to your housemate.

In a new study, researchers found that the genetics of a mouse's cage mate can affect its own health in a multitude of ways. Moreover, cage mates do this by influencing traits once thought to be controlled solely by an animal's own genes, such as growth rate and the functioning of its immune system .

"The take-away message here is that we need to pay attention to the genetic makeup of social partners , since in some cases it affects health more than the individual's own genes," said Amelie Baud, a postdoctoral fellow at the European Bioinformatics Institute in Hinxton, England, and first author on the study.

"This is something we did not know before," Baud told Live Science. "It means we need to stop looking at individuals in isolation and include social partners when we look at an individual's health."

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Scientists have long known that social interactions contribute to health and disease. For example, peer pressure can increase the chances that a teenager will take up smoking. However, the extent to which the genetic makeup of one animal can impact the traits of another that it lives with — an emerging concept called social or indirect genetic effects — is poorly understood.

In the new study, the researchers identified more than 40 traits in mice that a neighboring mouse's genetic profile may influence . They found that a cage mate's genetics contribute, on average, to about 10 percent of its partner's anxiety level, immune function, body weight, wound-healing speed and other traits.

The findings, published today (Jan. 25) in the journal PLOS Genetics, may apply to studies of complex traits in human populations, and further studies should look at this possibility, the researchers said.

Baud gave the example of a morning person living with a night owl . The morning person might develop an illness that's exasperated by a lack of sound sleep resulting from her staying up late with her partner. So, her partner's genetics — a natural inclination to stay up late — alters her own behavior and contributes to poorer health.

In the study on mice, however, the outcomes were not as obvious or explainable. For example, black mice housed with gray mice healed better than black mice housed with other black mice, but the researchers weren't sure why. Gray mice were less anxious when they were housed with black mice than when they are housed with gray mice. No type of mouse had universally positive or negative effects on its cage mates across all traits, the researchers said.

For some traits related to the immune system, social genetic effects accounted for nearly 30 percent of how genes were expressed, the study revealed.

The research was led by Oliver Stegle of the European Bioinformatics Institute, part of the European Molecular Biology Laboratory, which has labs in five European countries supported by 22 member states. Stegle's group aims to unravel how genetic background and environment jointly shape phenotypic traits — that is, how one's genes are expressed.

Baud said that her team's ongoing research "could inform patients and doctors on social contributions to disease and provide clues as to how to mitigate social influence, or indeed enhance it when it has beneficial effects ."

The findings highlight the fact that some important traits underlying health and disease appear to be beyond the individual and, instead, in the hands of one's partner, the researchers said.