Usable information on psychoactive substance use, abuse and dependence was available for both members of 1386 pairs and 11 single twins. The zygosity distribution of the complete pairs was: 220 male–male MZ, 117 male–male DZ, 448 female–female MZ, 263 female–female DZ and 338 male–female DZ.
We attempted to predict twin concordance for the 10 traits depicted in from our measures of childhood and adult environmental similarity. Of these 20 tests, three were significant at the 5% level, a pattern that does not differ significantly from chance expectation (Feild & Armenakis, 1974
). These results are consistent with prior studies of the equal environment assumption in studies of PSUD (Kendler & Prescott, 1998
; Kendler et al. 2000a
; Lynskey et al. 2002
Prevalence and tetrachoric correlations (with standard errors) for use of illicit psychoactive substances, symptoms of abuse/dependence and the diagnosis of abuse or dependence in Norwegian twins
Entry criteria for the CIDI PSUD section required lifetime use of illicit substances 10 or more times – which we term ‘significant use’. Thus, all of our results presented here are among individuals who screened positive for significant use. shows that significant use of illicit substances was relatively uncommon, being reported by only 6.4% of subjects. also reports twin resemblance for the drug use-related phenotypes assessed in this study. Resemblance for significant substance use was much higher in MZ twins (+0.80) than in DZ pairs (+0.16).
Any DSM-IV symptom of abuse or dependence with any substance was reported by 3.5% of the sample. The tetrachoric correlation for any symptom was substantially greater in MZ (+0.81) than in DZ pairs (+0.04). Full criteria for abuse or dependence of any substance were met by only 1.9% of the sample. Resemblance for a diagnosis of PSUD was much greater in MZ (+0.69) than in DZ pairs (+0.23).
Sufficient data were available to examine use, and symptoms or a diagnosis of abuse or dependence, only for cannabis. As seen in , the results closely paralleled those seen for any substance.
For stimulants, opiates, cocaine and psychedelics, prevalence rates were so low that we could only meaningfully examine lifetime use (). The prevalence of any use (among those with significant use) ranged from 0.9% for opiates to 2.7% for stimulants. Resemblance for lifetime use was for all substances higher in MZ than in DZ pairs.
depicts our model fitting results. Notably, for all substance use and abuse categories that we examined, we could, with considerable confidence, reject an ‘E-only’ model. Thus, we have evidence for the familial aggregation, within Norwegian twin pairs, of the tendency to use and abuse psychoactive substances. For all substance use and abuse categories, the AE model provided the best fit. For all the analyses, except for opiate and cocaine use, the CE model could be rejected with considerable statistical confidence. However, for opiate and cocaine use, the fit for the AE model was only slightly superior to that found for the CE model.
Model-fitting results for substance use, symptoms of abuse/dependence and the diagnosis of abuse or dependence
Parameter estimates and 95% confidence intervals (CIs) for the full (or ACE) model and the best-fit model are presented in . The estimated heritabilities in both the full and best-fit models are relatively high, exceeding 0.65 for all variables except opiate and cocaine use. The CIs for many of these estimates are relatively wide, reflecting the low accuracy of parameter estimation.
Parameter estimates (95% confidence intervals) for full and best-fit models for use of illicit psychoactive substances, symptoms of abuse/dependence and the diagnosis of abuse or dependence
Because of the rarity of the drug use and PSUD in this sample, these analyses were all conducted assuming no sex-dependent thresholds and no sex-dependent genetic and environmental parameters. However, of the 10 PSUD traits we examined, males had higher prevalence rates for all of them except opiate use. This difference reached statistical significance for any significant use [odds ratio (OR) 1.56, χ2 = 8.16, df=1, p=0.004], any diagnosis of abuse/dependence (OR 2.27, χ2 = 8.41, df=1, p=0.004), cannabis use (OR 1.56, χ2 = 8.00, df=1, p=0.005), cannabis abuse/dependence (OR 2.56, χ2=9.45, df=1, p=0.002), and any psychedelic use (OR 1.96, χ2=6.05, df=1, p=0.01). Therefore, for these five PSUD-related traits, we fitted twin models that allowed for both sex-dependent thresholds and parameters. In each case, the best AIC was obtained when the parameters were constrained to equality across the sexes and the thresholds permitted to differ in males and females. All parameter estimates from these models were within ± 0.02 of those presented in .