For externalizing problems, compared to offspring from control families (Group 4), offspring of an affected parent (Group 1) had higher scores, as did offspring of an unaffected parent with an alcoholic MZ co-twin (Group 2). Furthermore, Groups 1 and 2 did not differ in overall level of externalizing, with both groups differing significantly from offspring of an unaffected parent with an alcoholic DZ co-twin (Group 3). This pattern of Groups 1 and 2 > Groups 3 and 4 held in models without and with adjustment for offspring age and sex. The same pattern was observed for total problems: Groups 1 and 2 > Groups 3 and 4, regardless of covariate adjustment.
