There were small but significant shared environmental influences on both disorders as well as their co-morbidity, accounting for 3–6% of the variance and covariance, which corroborate the results from the meta analysis of twin studies on schizophrenia28. Twin studies have limited power to detect shared environmental effects, especially for binary traits29, and we suggest that caution is warranted when drawing to definite conclusion also for other psychiatric conditions when limited sample sizes are used.
