The onset of female reproductive potential (menarche) is an important stage in women’s development, resulting in both profound biological and socio-cultural change. Several studies have indicated that age at menarche is associated with father absence, in that females who do not live with their father in childhood reach reproductive age faster (Moffitt et al., 1992). These findings present an evolutionary puzzle as decreased parental investment, such as resulting from an absent father, ought to negatively impact offspring fitness, resulting in delayed reproduction. One set of adaptive explanations (Belsky et al., 1991) suggests that individuals alter their reproductive decision-making based on early-life adversity, such as father absence, as this is a cue for a high-mortality environment, in which case earlier reproduction may increase reproductive success. However, a recent alternative adaptive theory based on inclusive fitness considerations suggests that these father absence effects reflect the conflict between investing in one’s own reproduction (for direct fitness benefits) and investing in their mother’s reproduction (i.e., investing in siblings for indirect fitness benefits). Children are more likely to forego reproduction and invest in siblings if these are full siblings, due to higher relatedness coefficients, while with half- or step-siblings, which would be associated with father absence, children are more likely to invest in their own reproduction (Moya & Sear, 2014). To date, no tests of these competing hypotheses have been conducted. I therefore aim to explore the relative merits of each of these adaptive theories for the observed accelerated reproductive timing associated with father absence.