Birds of a feather flock together, is a proverbial English saying. For long it has been suspected to be true also for mental disorders, where a phenomenon called non-random mating is thought to occur. “Non-random mating” means that persons sharing a certain phenotype preferentially date and mate. IQ, height, or personality traits might be such phenotypes – and by the way, it is known that rather the “birds of a feather” model works for long-term relationships than the “opposite attracts” manner.
New research, published in JAMA Psychiatry, now sheds light on whether or not non-random mating is prominent in mental disorders (Nordsletten et al., 2016). The highly informative Swedish population registers were analysed to answer this question. As compared to matched controls, Swedes suffering from mental disorders had a significantly higher chance to have a partner who also suffers from a psychiatric illness. The effect was more pronounced within a certain diagnostic category than across disorders, if one considers all mental disorders. Interestingly, when non-psychiatric disorders such as rheumatoid arthritis or diabetes were studied, no hint for assortative mating was found, indicating that perhaps changed behavioral profiles underly non-random mating (assuming that e.g. diabetes does not influence your overall behavior to a large extend).
What happens if we look at individual psychiatric disorders? It appears that two clusters stick out; one being an “anxious cluster” with preferential mating between social phobia, OCD, agoraphobia and substance abuse (and also social phobia and autism); the other one being a neurodevelopmental cluster. This cluster shows a high degree of non-random mating between ADHD, autism and schizophrenia; actually, the highest level of assortative mating occurred within ADHD (correlation = 0.45 for both sexes), within autism (correlation of 0.48 and 0.45 for males and female index probands, respectively) and within schizophrenia (0.44 and 0.40). Also, between these disorders there were some meaningful correlations. It has to be noted that the number of ASD patients living in a relationship was rather low (less than 1000 males, equalling 5% of cases, and also less than 1000 females), also as compared to the number of mated patients suffering from schizophrenia.
While clinical experience since long tells us that ADHD patients tend to have a relationship with another ADHD patient, it is still highly interesting to actually see that in a large sample. Especially in ADHD, there is however considerable source for potential bias as also mentioned by the authors of the study; data came from hospital and psychiatric records, so that many ADHD patients might not have been picked up (as also evident from the size of the case sample of ca. 40,000 male and ca. 20,000 female ADHD patients). Thus, perhaps only a more severely affected sub-sample of ADHD patients might have entered the analysis.
The study has far-reaching implications for psychiatric genetics, as it suggests a higher rate of bilineal genetic transmission in families than expected by chance. This is an important caveat in linkage analyses and also next-generation sequencing approaches. However, beyond these scientific implications it also provides important clues for our clinical work: we should be aware that partners of our patients might be affected as well, which is further strengthening the relevance of psychoeducation for relatives. Also children of affected adults might be more troubled if both of the parents are affected with ADHD. The lesson we should learn is to look out for presence of mental disorders in the partners of our patients as well – in science, but also in the clinic.