Causes and consequences of adult sex ratio imbalance in a historical U.S. population

Abstract

The responsiveness of individuals to partner availability has been well-documented across the literature. However, there is disagreement regarding the direction of the consequences of sex ratio imbalance. Specifically, does an excess of males or females promote male-male mating competition? In an attempt to clarify the role of the adult sex ratio (ASR) on behaviour, here we evaluate both competing and complimentary expectations derived from theory across the social and biological sciences. We use data drawn from a historical, nineteenth century population in North America and target several life-history traits thought to be affected by partner availability: age at first birth, relationship status, completed fertility and longevity. Furthermore, we assess the role of various contributors to a population's ASR. We find that both the contributors to and consequences of sex ratio imbalance vary over time. Our results largely support predictions of greater male pairbond commitment and lesser male mating effort, as well as elevated bargaining power of women in response to female scarcity. After reviewing our findings, and others from across the literature, we highlight the need to adjust predictions in response to ASR imbalance by the: (i) culturally mediated mating arena, (ii) variable role of demographic inputs across time and place, (iii) constraints to behavioural outcomes across populations, and (iv) ability and accuracy of individuals to assess partner availability.This article is part of the themed issue 'Adult sex ratios and reproductive strategies: a critical re-examination of sex differences in human and animal societies'.

Adult sex ratios and reproductive strategies: a critical re-examination of sex differences in human and animal societies

“…For example, in humans, sociocultural norms governing reproduction can alter the costs and benefits to a certain behaviour (e.g. social sanctions against divorce) and accordingly its frequency [7]. Third, individuals vary and this can lead to substantial variation in behaviour within a population.…”

“…Schacht & Smith [7] and Uggla & Mace [55] make use of extensive, high-quality demographic data to explore multiple outcomes and heterogeneity of ASR effects across individuals. Specifically, Schacht & Smith [7] explore the consequences of a biased ASR for relationship and reproductive outcomes in a historical US population. They find that both the contributors to and consequences of ASR imbalance vary over time and, specifically, that indicators of male parental investment and female bargaining power are elevated in response to a surfeit of men.…”

Adult sex ratio and social status predict mating and parenting strategies in Northern Ireland

“…However, inconclusive or contradictory evidence that suggests the opposite relationship between the ASR and mating-related behaviours also exists, e.g. that a female-biased sex ratio might either be associated with earlier [11] or later [13] ages of women's first birth. With regards to violence, it has been found that male-male violence is lower when men are in excess [14], and that male mortality from violence and risk-taking is not higher when men are plentiful [15], challenging the idea that more men leads & 2017 The Author(s) Published by the Royal Society.…”

Persistent effect of sex ratios on relationship quality and life satisfaction

“…As a result, under such conditions, mating market models predict that women are more likely to marry, are less likely to participate in the labour force, and consume more leisure [6][7][8][9][10]. Evidence from traditional [11], historic [12] and industrialized [13] societies suggests that behavioural responses to imbalanced adult sex ratios can be both facultative and highly sensitive to ecological, economic and cultural context.…”

“…While these studies establish that sex ratios influence competition for mates, mate choice, pairing decisions and the relative power that members of each sex have to negotiate sex and relationship formation on favourable terms [3,4,12,14], it remains unclear whether these effects are transitory, lasting only until sex ratios return to normal, or are persistent. Some recent studies suggest that cultural changes concerning mating behaviour can persist for many generations after the circumstances that precipitated the change returned to 'normal'.…”