Pedigree-based analyses found that genetic differences account for 50–80% of the variation of observable (phenotypic) traits. Personality traits account for 34–48% of the variance being explained by genetic differences. Molecular genetic studies have also found that intelligence and personality variables are heritable by 30% and 0-15% respectively. These types of studies may differ because platforms operating genetic makeup are poor at tagging causal, low minor allele frequency, copy number and structural variants. A group genotyped for ~700,000 single-nucleotide polymorphisms (SNPs) was used in order to exploit the high levels of linkage disequilibrium (LD) found in people who are related to evaluate the total effect of genetic variants that are not observable in unrelated individuals. Genetic variants in low LD with SNPs in their genetic makeup explain more than half of the genetic variance in intelligence, education, and neuroticism. These additional genetic effects will aid in approximating the heritability estimates from twin studies for intelligence and education, but not for neuroticism and extraversion. Our ﬁndings are then replicated using assigned molecular genetic data from unrelated individuals to show that ~50% of differences in intelligence, and ~40% of the differences in education, can be explained by genetic effects when a larger number of rare SNPs are present. An evolutionary genetic perspective states that a large addition of rare genetic variants to individual differences in intelligence, and education is consistent with a balance of mutation and selection.

Extraversion and neuroticism are heritable, but are also linked to evolutionary fitness, which is a paradox. Intelligence and personality appear to be selective yet heritable at the same time. To solve this paradox, there must be rare variants playing a major role in the genetic addition to variance in these traits. We test whether genetic variants not in LD with genotyped single-nucleotide polymorphisms (SNPs) (including rare variants, copy number variants (CNVs) and structural variants) make an addition to intelligence and personality. Two different methods will be used to test this.

First method will include the use of an analytic design for combined pedigree and genome-wide molecular genetic data. This will be used to test whether rare genetic variants, CNVs, and structural variants add to the genetic difference in intelligence, neuroticism, and extraversion. Second method included unrelated participants and genotype data imputed using the Haplotype Reference Consortium (HRC) data. Minor allele frequency (MAF) was used to determine if this additional variance can also be recovered based on SNPs alone using imputation.

General intelligence has been found to be heritable with 50-80% being cause of genetic factors. However, less is known about the heredity of personality traits. Genetic heritability for extraversion and neuroticism have scored to be around 34-48%. However, molecular heritability scored to be only around 4-15% for neuroticism and 0-18% for extraversion.

For this study, 24,090 Scottish individuals were used and 23,919 donated a DNA sample for analysis. Some participants were related while others were unrelated and their education level was recorded. To analyse phenotypic data GREML-KIN was used and it included the analysis of genetic and similarity matrices. GREML-MS was used to analyse data in unrelated individuals and it focused on genetic variants.

The results have shown that when it comes to general intelligence pedigree genetic variants accounted for the majority of the total genetic contribution to phenotypic variation in these traits. These variants explained 28% of the variation in education and SNP effects explained 16%. Also, the genetic results have shown that general intelligence is likely to be heritable by 54% and education by 41%. As for neuroticism, it has been shown that additive common genetic effects explained 11% of the variance with pedigree-associated variants – 19%. Family similarity components only accounted for 2% of the variance at most. The results have shown that for extraversion common genetic variance accounts for 13% with family explaining 9% of phenotypic variation. The total contribution of all SNPs resulted in a heritability estimate of 50% for intelligence and 37% for education.

Reference: Nature, Molecular Psychiatry, https://doi.org/10.1038/s41380-017-0005-1