There was almost no continuity in common unique environmental effects. This is not surprising, given these effects are confounded with measurement error, which would have a time-specific impact. The impact of unique environment might also be expected to increase with age as twins spend more time outside of the family, yet the results revealed only a slight increase. This may be reflected in the increase in substance-specific genetic effects as choice of non-familial environments might be genetically influenced (Viken et al. 1999). However, for smoking and alcohol intoxication, specific unique environmental effects did increase with age.
