The conditional reproductive strategy theory highlights the importance of environmental influences on pubertal timing and suggests that girls use the presence or absence of their own father in the home as a clue to likely level of paternal investment that they can expect from their mate when they begin reproducing.  If their own father is absent, they learn that men are unreliable, whereas if their own father is present, then they learn to rely on men.  However, there is a piece missing from this explanation.

In order for the conditional reproductive strategy to evolve among women, men’s tendency toward forming committed relationships and making parental investment, in other words, how reliable men are as a source of material support, must be stable across generations.  The mother’s experience with her mates must be highly predictive of the daughter’s experience one generation later.  How could this be?

One suggestion is that girls use the presence or absence of the father in the home as an indicator of the institution of marriage in the society.  In this view, father absence signifies not necessarily the man’s willingness to form long-term relationships but a high degree of polygyny in society.  In a highly polygynous society, married men are spread thinly among their multiple wives, and they cannot spend much time with any one of their wives or their offspring.  Thus, the more polygynous the society, the less time any girl (or boy) spends with the father.  In contrast, in monogamous societies, married men have only one wife, so they can spend all of their time with their wife and children.  The degree of father absence might be a microlevel indictor (within the family) of a macrolevel degree of polygyny (within the society).

In polygynous societies, there is an incentive for girls to mature early because any pubescent girl can become a junior wife of a wealthy polygynist while a prepubescent girl cannot.  In contrast, there is no incentive for girls to mature early in monogamous societies because all adult men in such societies are already married (and cannot marry again), given a roughly 50-50 sex ratio, and pubescent girls can only marry young teenage boys, who do not have the wealth or status to support a family. 

Consistent with this logic, an analysis of cross-cultural data shows that girls undergo puberty significantly earlier in polygynous societies and in nominally monogamous societies with a high incidence of divorce (and thus a higher incidence of serial polygyny).  From this perspective, the average age of puberty has dropped precipitously in the United States in recent decades because the divorce rate (and thus the incidence of remarriage for men, that is, serial polygyny) has increased dramatically.  Recall that most Americans are in polygynous marriages.

The biochemical mechanism by which parental divorce precipitates early puberty in girls is not well known.  The evolutionary developmental psychologist Bruce J. Ellis at the University of Arizona suggests that pheromones (a chemical substance that travels from one individual to another in order for the former to influence the behavior of the latter) emitted by the stepfather and other unrelated men in the household might trigger early puberty in girls.  This is one of the remaining mysteries in evolutionary psychology.

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