In Dunedin, DNA was obtained from blood (93%) or buccal swabs (7%). In E-Risk, DNA was obtained from buccal swabs. Genotyping was carried out in the same laboratory, by a technician blind to data on maltreatment and depression. Genotyping of 5-HTTLPR was carried out following a standard protocol(Gelernter et al., 1997) with forward primer sequence 5′-ATGCCAGCACCTAACCCCTAATGT-3′ and reverse primer sequence 5′-GGACCGCAAGGTGGGCGGGA-3′. This amplifies a 419 base pair product for the 16 repeat (‘long’) allele and a 375 base pair product for the 14 repeat (‘short’) allele. PCR was carried out on a PTC-225 DNA engine (MJ Research). Products were separated on a 2.5% agarose gel (MultiABgarose, ABgene) supplemented with Ethidium bromide (0.03%, BDH) and visualised by ultraviolet transillumination. Genotypes were double called. Genotype frequencies for both samples were in Hardy–Weinberg equilibrium (Dunedin χ2(1)=1.913; p=0.1666; E-Risk χ2(1)=0.075; p=0.7835).
