The first menstrual period – known as menarche – is an important stage in women’s development and means that getting pregnant is possible. Although studies have suggested that factors such as genes, socioeconomic disadvantage and father absence are associated with menstruating at an early age, the reasons why some women reach menarche earlier, while others reach it much later, remain largely unclear. The study, published in Biology Letters, explored the idea that the benefits of investing in full siblings, known as inclusive fitness, may predict some of this variation.
Full siblings (children from the same mother and father) share more genes than half-siblings (children who share just one common parent) or step-siblings (children from both a different mother and a different father, and therefore share no genes). As genetic relatedness predicts who individuals cooperate with – we are more likely to help our kin than a random stranger – individuals with only full siblings may delay their reproduction to help their siblings.
The study used data from the Avon Longitudinal Study of Parents and Children (ALSPAC) cohort, based at the University of Bristol. As ALSPAC is a longitudinal birth cohort with detailed data on child development, the first menstrual period could be assessed in 'real-time' from a series of nine questionnaires, completed almost annually between the ages of eight and 17, and two research clinics attended at ages 13 and 16.
The research found that girls with only half- or step-siblings reached menarche on average five months earlier than those with only full siblings. Consistent with evolutionary predictions, individuals may therefore delay reproduction to invest in siblings who share a greater proportion of their genes.
Dr Daniel Smith from ALSPAC, said: "Previous studies have indicated that several factors, such as father absence or socioeconomic disadvantage, are associated with menstruating at an early age. These results suggest that inclusive fitness benefits to investing in full siblings, rather than father absence, may predict earlier menstruation."
These findings may also help understand a mystery in human evolution: why we have such a long period of childhood. The length of childhood in humans, extending well into teenage years, is much longer than in other species, yet the reasons for this remain unclear. This research suggests that one possible explanation may be that we have a long childhood because we delay reproduction to help our full siblings.