Genome-wide association studies have demonstrated that the genes involved in intelligence remain fairly stable over time. Specifically, in terms of IQ stability, "genetic factors mediated phenotypic stability throughout this entire period [age 0 to 16], whereas most age-to-age instability appeared to be due to non-shared environmental influences". These findings have been replicated extensively and observed in the United Kingdom, the United States, and the Netherlands. Additionally, researchers have shown that naturalistic changes in IQ occur in individuals at variable times. Influence of parents genes that are not inherited Kong reports that, "Nurture has a genetic component, i.e. alleles in the parents affect the parents' phenotypes and through that influence the outcomes of the child." These results were obtained through a meta-analysis of educational attainment and polygenic scores of non-transmitted alleles. Although the study deals with educational attainment and not IQ, these two are strongly linked. Spatial ability component of IQ Spatial ability has been shown to be unifactorial (a single score accounts well for all spatial abilities), and is 69% heritable in a sample of 1,367 pairs of twins from the ages 19 through 21. Further only 8% of spatial ability can be accounted for by shared environmental factors like school and family. Of the genetically determined portion of spatial ability, 24% is shared with verbal ability (general intelligence) and 43% was specific to spatial ability alone. Molecular genetic investigations A 2009 review article identified over 50 genetic polymorphisms that have been reported to be associated with cognitive ability in various studies, but noted that the discovery of small effect sizes and lack of replication have characterized this research so far. Another study attempted to replicate 12 reported associations between specific genetic variants and general cognitive ability in three large datasets, but found that only one of the genotypes was significantly associated with general intelligence in one of the samples, a result expected by chance alone. The authors concluded that most reported genetic associations with general intelligence are probably false positives brought about by inadequate sample sizes. Arguing that common genetic variants explain much of the variation in general intelligence, they suggested that the effects of individual variants are so small that very large samples are required to reliably detect them. Genetic diversity within individuals is heavily correlated with IQ. A novel molecular genetic method for estimating heritability calculates the overall genetic similarity (as indexed by the cumulative effects of