People with psychiatric disorders may be likely to marry and have children with other people who also have psychiatric disorders, according to a new study from Sweden.

The study did not examine why people with psychiatric conditions, such as schizophrenia and depression, may tend to mate with other people with such conditions, and therefore, the mechanisms behind this phenomenon are not clear, said Ashley E. Nordsletten, a co-author of the study and a postdoctoral research fellow at Karolinska Institutet in Stockholm.

But one possible reason is that people may simply select partners who share certain traits with them, she told Live Science.

In the study, the researchers looked at the health data from about 700,000 people who were admitted to Swedish hospitals between 1973 and 2009. This population included more than 70,000 people with schizophrenia, people with 10 other major psychiatric disorders, and people with chronic physical illnesses such as Crohn’s disease, diabetes or multiple sclerosis.