Finally, DD was significantly associated with family SES derived from parental occupation data: F1,644 = 10.47, P = 0.001 at age 12 and F1,531 = 11.86, P = 0.001 at age 14, although effect sizes were modest (d = .27 and .35, respectively). Specifically, at both ages participants from families with lower SES tended to prefer smaller but immediate rewards. SES was included as a covariate (definition variable) in the genetic analysis through the use of four dummy variables (with the highest income bracket as the comparison group). Percentages of respondents in the lowest through highest socioeconomic brackets were 16, 26, 25, 17, and 15% respectively (1% of respondents had not socioeconomic data and were set to missing for these variables).
