Similar analyses of a number of other complex traits, for example, human height,43 crystallised and fluid intelligence,57 Crohn's Disease, Bipolar Disorder and Type 1 Diabetes,58 Body Mass Index and biological measures such as von Willebrand Factor and QTi51 have shown that common SNPs explain a substantial part of their reported heritabilities. If rarer variants are more likely to affect variance of neuroticism and extraversion, compared with the traits described above, relatively low heritability estimates would be expected from the current design (that is, genome-wide SNP analysis). A potential reason for a relatively large contribution of rare variants is the possible relation between neuroticism/extraversion and fitness. If personality traits neuroticism and extraversion are correlated with fitness, and decreased reproductive fitness reduces the frequencies of risk variants for less desirable personality traits, one would expect a relatively large contribution of rare variants to standing genetic variation for these traits. As the current data set primarily consists of SNPs with MAF >0.01, rare (causal) variants are generally poorly tagged which, in turn, would explain some of the remaining heritability. Plausible arguments can be
