Table 2 presents descriptive statistics of the studies weighted by the number of sibling pairs per study. Average age at baseline and follow-up assessment was in the 20s with sizeable standard deviations. The studies covered nearly the entire range of the lifespan beginning at 1 year of age and ending with 86 years. The average time interval between assessments was 5.43 years. A little more than half of the participants were female. The racial/ethnic composition of the samples was not well-reported in the articles. Based on the location and populations sampled (e.g., Minnesota; Northern Europe), we can infer that the twin studies tended to be predominantly White. The estimate given in Table 2 is likely to be an overestimate of the minority representation in the samples because the Southern California Twin Project is a large outlier in terms of racial composition with over 70% minority participants (Niv, Tuvblad, Raine, Wang, & Baker, 2012). This project contrasts with the large scale European studies in Finland (Viken, Rose, Kaprio, & Koskenvuo, 1994) and the Netherlands (Rietveld, Hudziak, Bartels, van Beijsterveldt, & Boomsma,
