Background: Five DNA markers (single-nucleotide polymorphisms, SNPs) have recently been found to be associated with general cognitive ability "g") in a sample of 7414 7-year-old twins. These children have also been studied at 2, 3, 4, and 7 years of age on measures of cognitive and language development and behaviour problems; family environment was also assessed. Methods: We used these data to conduct a behavioural genomic analysis of the five SNPs and a composite of them ("SNP set") that explored developmental, multivariate, and genotype-environment (GE) issues. Results: The "g" SNP set identified at 7 years yielded significant associations with "g" as early as 2 years. In multivariate analyses at 7 years, the "g" SNP set was more strongly associated with verbal than nonverbal ability and with reading more than mathematics performance. GE correlations were found between the SNP set for "g" at 7 years and preschool proximal measures of the family environment (chaos and discipline) rather than distal measures (maternal education and father's occupational class), suggesting evocative rather than passive GE correlation. Significant GE interactions were found for discipline, education and occupation in which the association between the SNP set and "g" at 7 years is stronger in low-risk environments. Conclusions: Although the effect sizes of the five SNP associations are very small, behavioural genomic analyses using a "g" SNP set illustrate how developmental, multivariate and GE questions can be addressed as more DNA associations are identified for complex traits such as "g".