Table 1.

Intra-pair twin correlations for delay discounting measures and heritability estimates based on the final bivariate Cholesky model.

DD Measure	Age	Twin correlations		Proportions of variance
		rmz (n)	rdz (n)	a2 (95% CI)	e2 (95% CI)
AUC	16	.45* (50)	.16 (39)	.46 (.28 – .62)	.54 (.38 – .72)
	18	.64* (113)	.27* (125)	.62 (.51 – .70)	.38 (.30 – .49)
log k	16	.40* (50)	.03 (38)	.35 (.17 – .52)	.65 (.48 – .83)
	18	.57** (108)	.23* (124)	.55 (.43 – .66)	.45 (.34 – .57)
AUC (Ex)	16	.39* (39)	.16 (28)	.35 (.14 – .56)	.65 (.44 – .86)
	18	.64** (69)	.31* (82)	.63 (.50 – .73)	.37 (.27 – .50)
log k (Ex)	16	.34 (32)	−.06 (28)	.21 (.05 – .43)	.79 (.57 – .95)
	18	.61** (68)	.27* (82)	.62 (.47 – .73)	.38 (.27 – .53)

AUC is area under the discounting curve; log k is log₁₀ of the k-coefficient estimated by fitting a hyperbolic model to individual data; “Ex” indicates re-analysis of data after the exclusion of subjects with “nonsystematic” response patterns. r_mz and r_dz are intrapair twin correlations showing the degree of twins’ resemblance with respect to DD measures (numbers of twin are shown in brackets); a²=proportion of variance in DD measures attributable to additive genetic influences (heritability); e²=proportion of variance attributable to non-shared environmental influences including measurement error (95% confidence intervals are shown in brackets).

Significance of twin correlations:

^*p<.05;

^**p<.01.