Background: The differential utility of neurocognitive impulsivity and externalizing/
internalizing traits as putative endophenotypes for dependence on heroin vs. amphetamine is unclear.
Objective: This exploratory study aims to determine: (1) whether neurocognitive impulsivity dimensions
and externalizing/internalizing traits are correlated between siblings discordant for heroin and amphetamine
dependence; and (2) which of these associations are common across substances and which are substance-
Methods: Pearson correlations between individuals with ‘pure’ heroin and amphetamine dependence and
their unaffected biological siblings (n = 37 heroin sibling pairs; n = 30 amphetamine sibling pairs) were
run on 10 neurocognitive measures, 6 externalizing measures, and 5 internalizing measures. Sibling pair
effects were further examined using regression.
Results: Siblings discordant for heroin dependence were significantly correlated on delay aversion on the
Cambridge Gambling Task, risk-taking on the Balloon Analogue Risk Task, sensation seeking, and hopelessness.
Siblings discordant for amphetamine dependence were significantly correlated on the quality of
decision-making on the Cambridge Gambling Task, discriminability on the Immediate Memory Task,
commission errors on the Go/No Go Task, trait impulsivity, ADHD and anxiety sensitivity.
Conclusion: Dimensions of impulsivity and externalizing/internalizing traits appear to aggregate among
siblings discordant for substance dependence. Risk-taking propensity, sensation seeking and hopelessness
were specific for heroin sibling pairs. Motor/action impulsivity, trait impulsivity, and anxiety sensitivity
were specific to amphetamine sibling pairs. Decisional/choice impulsivity was common across both heroin
and amphetamine sibling pairs. These findings provide preliminary evidence for the utility of neurocognitive
impulsivity and externalizing/ internalizing traits as candidate endophenotypes for substance
dependence in general and for substance-specific dependencies.