CHRM2 gene shows highly significant within-family association with intelligence. rs324650 T allele increases PIQ by 4.6 points and FSIQ by 4.1 points. All three CHRM2 SNPs (rs2061174, rs324640, rs324650) significantly associate with PIQ. Heritability of PIQ reaches 73% in young cohort and 71% in adults. VIQ heritability 70-78% and FSIQ 78-80% across cohorts. Population-based associations attenuated due to stratification masking true effects. Within-family effects 1.5-2.5 times larger than population effects. TT genotype at rs324650 yields 9.2 PIQ point advantage over AA. Increaser alleles consistently produce higher IQ means across genotypes in both cohorts. CHRM2 regulates neuronal excitability, synaptic plasticity, and acetylcholine feedback linked to cognition. Family-based tests guard against false negatives from stratification. Study confirms prior CHRM2-IQ link with stronger effects using tagging SNPs. No shared environment explains IQ variance – only additive genetics and unique environment. Dutch twin families provide representative population sample.