Total unique environmental effects revealed a consistent trend to increase in importance with age. Smoking-specific unique environmental effects also increased in importance whereas common unique environmental effects remained fairly consistent across development. Of note, at age 13–14 all of the unique environmental variance was accounted for by the common factor. A small amount of cross-time continuity for specific unique environmental effects was shown between ages 13–14 and 16–17 and ages 16–17 and 19–20.
