A study by JAMA Psychiatry of families who live in Sweden reveals that the risk of tic disorders, including Tourette syndrome as well as chronic tic disorders, increases with degrees of genetic relatedness.

Tic disorders are commonly thought to pass through a family, but not enough research has been done about heritability and the estimates of familial risk. Dr. David Mataix-Cols, of the Karolinska Institutet in Stockholm, along with co-authors of the study estimated family clustering and heritability of tic disorders by using data from two Swedish population-based registers. They were able to identify 4,826 people diagnosed with having Tourette syndrome or chronic tic disorders dating back from 1969 to 2009. (72.8 percent of those patients had at least one lifetime psychiatric co-existing condition.)

First-degree relatives of the individuals with a tic disorder were found to have a higher risk of having either Tourette syndrome or chronic tic disorder than second- or third-degree relatives. Whether parents or children of the individuals with a tic disorder, full siblings had a 50 percent genetic similarity as well as a shared environment which they grew up in, and were at a higher risk than maternal half siblings. Half siblings shared a 25 percent genetic similarity and spent time in a shared environment with the individuals who had a tic disorder. First cousins, who shared a 12.5 percent genetic similarity, still had a three-fold higher risk of having either chronic tic disorder or Tourette syndrome compared with control patients.

Although this is data that has been gathered for the study, the authors of the study are quick to say that the population they gathered is just a fraction of the Swedish population, and of the population of the world. That being said, the results of the study may not be generalizable to other populations. The Centers for Disease Control and Prevention says that one out of every 360 children aged 6 through 17 years of age are affected with Tourette syndrome, which is typically the time frame that symptoms begin to show themselves. Boys are affected three to five times more than girls are.

“The heritability of tic disorders was estimated to be approximately 77 percent,” the study concluded. “Tic disorders, including TS and CTDs, cluster in families primarily because of genetic factors and appear to be among the most heritable neuropsychiatric conditions.”

Source: Mataix-Cols D. JAMA Psychiatry. 2015.