They say the longer you’re married and live together, the more you begin to look alike and adopt one another’s mannerisms. Now, new research suggests that this may have some scientific backing — people who cohabit and raise a child together experience changes in their immune systems on a cellular level, in which they become more similar to their partner’s and child’s.

The human immune system is in many ways an autobiography of your life, molded by not only genes but also your childhood environment, diet, lifestyle, and infections that have left their mark in antibodies. Research has shown that chronic pain, stress, and anxiety can all change your immune system on a cellular level. In fact, the NIH states that the immune system is actually defined more by the environment than by your genes; explaining that it’s a fluid, always-changing entity that must “continually [adapt] to its encounters with hostile pathogens, friendly gut microbes, nutritional components, and more.”

The new study, conducted by researchers at VIB and KU Leuven in Belgium and the Babraham Institute in the U.K. — wanted to examine how two people living in very close contact experienced changes in their immune systems. The researchers examined some 670 people between the ages of two and 86 over the course of three years, measuring how age, gender, obesity, and other factors impacted their immune systems.

They found that the single most influential factor was whether a person was co-parenting a child with someone else under the same roof (and typically married), leading to a 50 percent lower amount of variation between the partners’ immune systems compared to other individuals. Cohabitation among married people was even more impactful on the immune system than gastroenteritis, leading the researchers to believe that living together even among non-married couples would have the same effect.

“[T]he largest influence on immunological variation identified was cohabitation, with 50 percent less immunological variation between individuals who share an environment (as parents) than between people in the wider population,” the authors wrote. “These results identify local environmental conditions as a key factor in shaping the human immune system.”

It makes sense that moving in with your partner would have a significant impact on your body. Syncing up your sleep schedules, eating similar foods, being exposed to the same pathogens or pollution, catching each other’s viruses, and exchanging millions of good and bad bacteria through kissing or other forms of contact are just some of the ways you'd begin to swap immune and microbiome features. In 2013, researchers found that families who lived together shared more similar gut microbiomes.

In addition, the researchers posit that raising a child significantly impacts the bodies of parents. “Since parenting is one of the most severe environmental challenges anyone willingly puts themselves through, it makes sense that it radically rewires the immune system — still, it was a surprise that having kids was a much more potent immune challenge than severe gastroenteritis,” said Dr. Adrian Liston, a researcher at VIB and KU Leuven and an author of the study, in the press release. “That’s at least something for prospective parents to consider — the sleep deprivation, stress, chronic infections and all the other challenges of parenting does more to our body than just gives us gray hairs.”

Source: Carr E, Dooley J, Garcia-Perez J, Lagou V, Lee J, Wouters C. The cellular composition of the human immune system is shaped by age and cohabitation. Nature Immunology, 2016.