Blessed are the meek for they shall inherit the earth: quietness component of introversion is associated with single nucleotide polymorphisms in two circadian clock genes

Individual differences studied by chronobiologists and personality psychologists are usually shaped by polygenic selection occurring by small allele frequency shifts spreading across many loci. Therefore, the candidate gene association studies suffer from increased likelihood of false positive findings. We previously associated a PER3 single nucleotide polymorphism (SNP, rs228697) with self-ratings on personality-relevant nouns exemplifying personality dimension of Extraversion/Introversion in a sample of 88 female students. To replicate and extend this finding, we genotyped three more SNPs in three circadian clock genes. The results indicated that the minor alleles of PER3 rs228697 and PER2 rs934945 were rather similar in terms of their association with a personality type nicknamed “demure persona” (i.e. described by such nouns as “quietness”, “restraint”, “taciturnity”, “bashfulness”, “timidity”, “constraint”, and “reticence”). Analysis of data from populations of the 1000 Genomes Project suggested that, like frequencies of the minor alleles of many SNPs in circadian clock genes, the frequencies of these two SNPs were higher in populations of out-of-African ancestry compared to populations of African ancestry. We suggested that genetic candidates for Extraversion/Introversion can be prioritized in future association studies by means of identification of genetic signatures of polygenic selection imposed by out-of-Africa expansion of ancestral populations.