Breastfeeding is considered the best and most natural way of feeding infants during the first months of life. Breastfeeding has multiple short- and long-term benefits for the health of the mother and babies, and from an evolutionist standpoint, it would be a behavior worth preserving throughout time. The aim of the present study was to explore the relative influence of genetic and environmental factors in this behavior. Three hundred and ninety pairs of adult female twins provided information about whether they breastfed their children and for how long. Three variables were analyzed: initiation and duration for the first baby, and mean duration for the complete offspring. Polychoric correlations were consistently higher for monozygotic twins, supporting a role for genetic factors (0.49 vs. 0.22 for initiation; 0.44 vs. 0.22 for duration in the first newborn; and 0.52 vs. 0.31 for duration on average). Model-fitting analyses found that in the best-fitting model, variance was explained by additive genetic and non-shared environmental factors, with estimated heritabilities ranging from 0.39 to 0.52 in the measures studied. The rest of the variance would be due to unique environmental factors. We conclude that genetic factors have a significant impact on the complex behavior of breastfeeding.