In a major scientific discovery this week, it turns out that, just like cancer, red hair, and a vestigial tail, a fondness for cannabis is inherited through your genes. At least, that’s part of the reason.

A study published this week in Nature identifies 21 genes in the human genome which show variances between cannabis users and non-cannabis users.

This is part of an ongoing process in discovering the pothead genome. Some of these genes have been identified before, but many had not. So far only 11% of the variance between stoners and the 420-unfriendly have been identified, as reported by Fortune.

The study also found genes which link cannabis use to other inherited traits. These include risk-taking, alcohol consumption, smoking cigarettes, reproductive success, and, maybe most interestingly, schizophrenia.

The genetic link to schizophrenia throws shade on former accusations that cannabis use could actually cause an increased likelihood for developing the mental illness. What this study shows instead is that people born with a likelihood for developing schizophrenia are also sometimes born with a likelihood for cannabis use. However, people with mental problems such as schizophrenia might also be likely to self-medicate with cannabis use.

“This suggests that individuals with schizophrenia have a higher risk to start using cannabis,” the paper reads. “Our findings may indicate that individuals at risk for developing schizophrenia experience prodromal [that is, early] symptoms or negative affect that make them more likely to start using cannabis to cope or self-medicate.”

Researchers came to their findings by compiling genetic data from the ancestry site 23andme, the International Cannabis Consortium, and the United Kingdom biobank. The study’s sample size was roughly 185,000 volunteers, which is five times as large as any other similar study to date.

Photo via Flickr user Stuart Cale