detect genetic variants of small effect size (<1% of the variance explained). The aim of the current study was to employ meta-analytic techniques to evaluate the possibility of small genetic effects on the TCI. By combining four cohorts totaling over 11 000 persons, this sample has 80% power to identify association with an SNP responsible for as little as 0.4% of the variance in the temperament scales, at an alpha level of 1.2 × 10−8 (a genome-wide significance level of 5 × 10−8 corrected for testing four temperament traits). Additionally, two features of the study design should minimize the degree of phenotypic and genotypic heterogeneity across the cohorts. First, temperament in all four cohorts was analyzed using identical items. Second, three of the four cohorts were derived from Finland, a relatively genetically homogeneous population.
