The genetics of impulsivity: evidence for the heritability of delay discounting

Abstract

Background: Delay discounting (DD), a decline in the subjective value of reward with increasing delay until its receipt, is an established behavioral model of impulsive choice, a key component of a broader impulsivity construct. Greater DD, i.e., a tendency to choose smaller immediate over larger delayed rewards, has been implicated as a potential intermediate phenotype (endophenotype) for addictive disorders and comorbid externalizing psychopathology, particularly in adolescence. However, genetic and environmental origins of DD remain unclear. Accordingly, the goal of the present study was to assess heritability of DD, an important aspect of its utility as an endophenotype.

Methods: A commonly used computerized procedure involving choice between varying amounts of money available immediately and a standard amount of $100 presented at variable delays was administered to a population-based sample of twins aged 16 and 18 (n = 560, including 134 monozygotic and 142 dizygotic pairs). DD was quantified using area under the discounting curve and the k coefficient estimated by fitting a hyperbolic model to individual data. Heritability was assessed using linear structural equation modeling of twin data.

Results: The genetic analysis revealed significant heritability of both DD measures (area under the discounting curve: 46% and 62%; k: 35% and 55% at age 16 and 18, respectively).

Conclusions: The present study provides evidence for heritability of both model-based and model-free DD measures and suggests that DD is a promising intermediate phenotype for genetic dissection of impulsivity and externalizing spectrum disorders.

Keywords: Decision making; Delay discounting; Endophenotype; Heritability; Reward; Twins.

Figure 1

Scatterplots of intra-pair twin correlations for delay discounting in monozygotic (MZ) and dizygotic (DZ) twin pairs. Each circle represents a twin pair, such that delay discounting value of Twin 1 (areaunder the discounting curve) is plotted against the corresponding value of Twin 2. Solid lines indicate linear regression of Twin 1 on Twin 2. Broken lines indicate 95% confidence intervals.

Figure 2

Unstandardized parameter estimates for the final model for delay discounting (area under the curve) at ages 16 and 18 years. A=additive genetic influences, E=nonshared environmental influences. Only one twin is shown for simplicity. Parameter estimates were equated for members of a twin pair, with the additive genetic factors connecting the members of a twin pair. The path connecting MZ twins was fixed to 1.0 and the path connecting DZ twins was fixed to 0.5, per quantitative genetic theory.