Objectives: Breast-fed C-allele carriers of the rs single nucleotide polymorphism in the fatty acyl desaturase 2 ("FADS2") gene have been reported to show a 6.4 to 7 IQ point advantage over formula-fed C-allele carriers, with no effect of breast-feeding in GG carriers. An Australian sample was examined to determine if an interaction between breast-feeding and the rs174575 single nucleotide polymorphism had any effect on IQ. Method: This hypothesis was tested in more than 700 families of adolescent twins assessed for IQ and breast-feeding, birth weight, and "FADS2" polymorphisms, and parental socioeconomic status and education, and maternal "FADS2" status. Results: No significant evidence for a moderating effect on IQ of rs174575 C-carrier status and breast-feeding was found, and there no effects of maternal "FADS2" status on offspring IQ. In addition, no main effects of any "FADS2" polymorphisms on IQ were found when the genotype was kept as two-homozygote and one-heterozygote categories and indeed no evidence for effects of breast-feeding on IQ scores after controlling for parental socioeconomic status and education. The investigation was extended to two additional "FADS2" polymorphisms (rs1535 and rs174583), but again, although these polymorphisms code alleles affecting fatty acid metabolism, no main or interaction effects were found on IQ. Conclusion: These results support the view that apparent effects of breast-feeding on IQ reflect differential likelihood of breast-feeding as a function of parental education and did not support the predicted interaction effect of "FADS2" and breast-feeding on IQ. (Contains 6 tables and 1 figure.)