The design argument for the existence of God took a probabilistic turn in the 17 th and 18 th centuries. Earlier versions, such as Thomas Aquinas' 5 th way, usually embraced the premise that goal-directed systems (things that "act for an end" or have a function) must have been created by an intelligent designer. This idea – which we might express by the slogan "no design without a designer" – survived into the 17 th and 18 th centuries, 1 and (...) it is with us still in the writings of many creationists. The new version of the argument, inspired by the emerging mathematical theory of probability, removed the premise of necessity. It begins with the thought that goal-directed systems might have arisen by intelligent design or by chance; the problem is to discern which hypothesis is more plausible. With the epistemic concept of plausibility characterized in terms of the mathematical concept of probability, the design argument was given a new direction. The new probabilistic perspective did not extinguish the older idea of "no design without a designer." The two conflicting approaches coexisted, often with less than perfect clarity about their fundamental difference. At the same time, the details of how the probabilistic perspective ought to be articulated were slow in emerging. These characteristics of uneven development are exemplified in a debate that took place in the 18 th century among three probabilists – John Arbuthnot, Nicholas Bernoulli, and Abraham DeMoivre – on the proper explanation of human sex ratio. Arbuthnot proposed a probabilistic version of the design argument, claiming that it is intelligent design, not chance, that provides the better explanation of why slightly more boys than girls are born each year. Bernoulli rejected Arbuthnot's argument. DeMoivre defended Arbuthnot against Bernoulli's criticisms. The problem of explaining sex ratio, both in human populations and in the rest of nature, experienced another transformation after 1859 --it became a problem for the theory of evolution by natural selection. Darwin (1871) addressed the question in the first edition of The Descent of Man but withdrew his suggestion in the second (Darwin 1874). Carl Düsing (1884) provided a mathematical model that moved beyond the explanation Darwin offered. Fisher (1930) added the new idea of parental expenditure. Williams (1966) argued that the sex ratios found in nature provide the opportunity to test hypotheses of group selection against hypotheses of individual selection. Hamilton (1967) constructed a model that generalizes Fisher's approach and represents the effects of both group and individual selection. The theory of natural selection gradually developed the ability to predict sex ratios, not just explain them post hoc. (shrink)

This paper addresses the question, which sex ratio will evolve in a population that is subject to mutation and drift. The problem is analyzed using a simulation model as well as analytical methods. A detailed simulation model for the evolution of a population's allele distribution shows that for the sex ratio game a wide spectrum of different population states may evolve from on the one hand a monomorphic state with one predominant allele and with all other alleles suppressed by the (...) forces of selection, to on the other hand a polymorphism determined by recurrent mutations. Which of these states will evolve depends on the population size, the mating system and the rate of mutations. For the sex ratio game the evolutionarily stable strategy (ESS), as defined by evolutionary game theory, can only predict the population sex ratio but not the underlying stable population state. A comparison of different approaches to the problem shows that false predictions of the stable population states might result from two simplifying assumptions that are fairly common in evolutionary biology:a) it is assumed that mutations are rare events and there is never more than one mutant gene present in a population at any one time; b) a deterministic relationship is assumed between the fitness assigned to an individual's strategy and the individual's contribution to the gene pool of future generations. (shrink)

Can mammalian mothers adaptively control the sex of their offspring? The influential Trivers-Willard hypothesis proposes that when maternal condition increases the fitness of sons more than that of daughters, the proportion of sons produced should increase with maternal condition. Studies of mammals, however, often fail to support this hypothesis. This article highlights recent advances, including studies on the assumptions of the TWH and physiological mechanisms for sex-ratio manipulation. Particular emphasis is placed on how factors such as paternal quality, maternal reproductive (...) costs and environmental conditions experienced by mothers early in life can mask/alter the expected relationship between maternal condition and offspring sex ratio or lead to apparent support for the TWH. While there is growing evidence that sex ratio around conception may be maternally and paternally manipulated, a challenge for future studies on sex allocation is to integrate how multiple and potentially opposite selective pressures affect offspring sex ratio. The relationship between offspring sex and maternal condition lies at the heart of one of the most controversial hypotheses in evolutionary biology. This review highlights recent findings on the Trivers-Willard hypothesis with particular emphasis on factors which may mask the relationship of interest or falsely demonstrate it. (shrink)

Different types of explanations coexist in present-day biology. Functional explanations describe mechanisms, whereas evolutionary explanations provide answers to the question “why?” mostly by appealing to the past and present action of natural selection. But the relations between these two types of explanations, as well as the relative insights they offer, vary from one domain of research to another. We will illustrate this complex landscape of biological explanations with three examples involving aging, the sex ratio, and the phenomenon of genomic imprinting. (...) We will show that the two types of explanations have recently often progressed towards each other. In consequence, they cease to be “pure” functional or evolutionary explanations and become explanations that may be alternatively considered as functional or evolutionary. (shrink)

Secular change in sex ratios is examined in relation to experience in the family. Two theoretical perspectives are outlined: Guttentag and Secord’s (1983) adaptation of social exchange theory, and sexual selection theory. Because of large-scale change in number of births and typical age differentials between men and women at marriage, low sex ratios at couple formation ages existed in the U.S. between 1965 and the early 1980s. The currently high sex ratios, however, will persist until the end (...) of the century. High sex ratios appear to be associated with lower divorce rates, male commitment to careers that promise economic rewards, male willingness to engage in child care, higher fertility, and higher rates of sexual violence. Sexual selection theory calls attention to intrasexual competition in the numerically larger sex. (shrink)

Independent samples of women were surveyed to test Trivers and Willard’s hypothesis that the mother’s condition and her ability to invest in her offspring affect the (secondary) sex ratio of her offspring. Patterns of sex ratios (number of males per 100 females) were analyzed in conjunction with four attributes of a mother’s microenvironment: level of health in her community, family structure, relative access to resources, and her birthing history. The results inferentially support the hypothesis that the microenvironment of the (...) woman would act to bias the sex ratio of her offspring. These specific data lend support to Trivers and Willard’s general hypothesis. (shrink)

Sex allocation research has primarily focused on offspring sex‐ratio adjustment by mothers. Yet, fathers also benefit from producing more of the sex with greater fitness returns. Here, we review the state‐of‐the art in the study of male‐driven sex allocation and, counter to the current paradigm, we propose that males can adaptively influence offspring sex ratio through a wide variety of mechanisms. This includes differential production and motility of X‐ versus Y‐bearing sperms in mammals, variation in seminal fluid composition in haplo‐diploid (...) invertebrates, and epigenetic mechanisms in some fish and lizards exhibiting environmental sex determination. Conflicts of interest between mothers and fathers over offspring sex ratios can emerge, although many more studies are needed in this area. While many studies of sex allocation have focused on adaptive explanations with little attention to mechanisms, and vice versa, the integration of these two topics is essential for understanding male‐driven sex allocation. (shrink)

We hypothesize that under the predicted scenario of climate change epigenetically mediated environmental sex determination could become an epigenetic trap. Epigenetically regulated environmental sex determination is a mechanism by which species can modulate their breeding strategies to accommodate environmental change. Growing evidence suggests that epigenetic mechanisms may play a key role in phenotypic plasticity and in the rapid adaptation of species to environmental change, through the capacity of organisms to maintain a non‐genetic plastic memory of the environmental and ecological conditions (...) experienced by their parents. However, inherited epigenetic variation could also be maladaptive, becoming an epigenetic trap. This is because environmental sex determination can alter sex ratios by increasing the survival of one of the sexes at the expense of negative fitness consequences for the other, which could lead not only to the collapse of natural populations, but also have an impact in farmed animal and plant species. (shrink)

Sex ratios have widely been recognized as an important link between demographic contexts and behavior because changes in the ratio shift sex-specific bargaining power in the partner market. Implicitly, the literature considers individual partner market experiences to be a function of local sex ratios. However, empirical evidence on the correspondence between subjective partner availability and local sex ratios is lacking so far. In this paper, we analyzed how closely a set of different local sex ratio measures correlates (...) with subjective partner market experiences. Linking a longitudinal German survey to population data for different entities, we used multilevel logistic regression models to explore associations between singles’ subjective partner market experiences and various operationalizations of local sex ratios. Results suggest that local sex ratios correlated only weakly with subjective partner market experiences. Adult sex ratios based on broad age brackets, including those for lower-level entities, did not significantly predict whether individuals predominantly met individuals of their own sex. More fine-grained, age-specific sex ratios prove to be better predictors of subjective partner market experiences, in particular when age hypergamy patterns were incorporated. Nevertheless, the respective associations were only significant for selected measures. In a complementary analysis, we illustrate the validity of the subjective indicator as a predictor of relationship formation. In sum, our results suggest that subjective partner availability is not adequately represented by the broad adult sex ratio measures that are frequently used in the literature. Future research should be careful not to equate local sex ratios and conscious partner market experiences. (shrink)

Operational sex ratio (OSR) is the correct sex ratio measure for predicting sociosexuality, but it is unclear whether this is the measure used. It would be valuable to know how OSR and sociosexuality correlate separately for males and females. The relationship between sociosexuality and OSR should also be examined with OSR measured at the local level of the mating market, where sex ratio must be having its psychological effects.

The male gametes of most organisms lack cytoplasm. Consequently, most cytoplasmic genetic elements are maternally inherited: they cannot be transmitted patrilinnearly. The evolutionary interests of cytoplasmic elements therefore lie in transmission through the female. These elements may thus be in evolutionary conflict with nuclear genes which are transmitted by both sexes. This conflict is manifested in observations of cytoplasmically induced biased sex‐ratios. Some cytoplasmic genes avoid this fate by biasing the primary sex ratio towards females, or by inducing parthenogenesis. (...) Others kill male hosts, and either achieve transmission via dispersal, or benefit their clonal relatives in the dead male's female siblings. Still others cause the failure of zygotes resulting from pairings between males carrying specific microbes and females lacking them, causing an increase in the microbes through the sterilisation of non‐bearing females. Many, but not all, of these ‘ultra‐selfish’ microbes are closely related. Investigations of the significance of their phylogenetic affinities, or lack of them, their adaptability in terms of the methods by which they avoid, or ameliorate, the adverse effects of being in male hosts, and their importance as selective agents in the evolution of invertebrate sex determination systems, provide fertile spheres for future research. (shrink)

This article examines changes and continuities in the epistemic and methodological presence of “race” in British imperial demography from 1920 to 1960. It does so in relation to population-level interventions aimed at improving reproduction in the New Hebrides. Through an examination of the sex ratio in relation to debates about demographic decline, the article describes aspects of how sexual selection was connected to race thinking. Taking a balanced sex ratio as a marker of well-adapted, healthy populations—biologically and culturally—the British authorities (...) in the New Hebrides attempted to regulate the bride price in an attempt to level the imbalanced sex ratio. They believed that this intervention would reduce the marriage age of men while also appeasing missionary agendas of changing marriage and kinship practices. I use the metaphor of a “watermark” to think through the conceptual and methodological absent/presence of race in colonial demography and colonial administrators’ attitudes toward and interventions in local reproductive practices. (shrink)

Using Australian records of human births for the first six decades of the twentieth century, the proportions of male and female births are studied in relation to the mother's age, the father's age, the parents' ages in combination, and certain other factors. The principal data used relate to the years 1931–55, during which over 3 million children were born, yet even so the significant features are not easy to discern.

Sex selection of children by pre-conception and post-conception techniques remains morally controversial and even illegal in some jurisdictions. Among other things, some critics fear that sex selection will distort the sex ratio, making opposite-sex relationships more difficult to secure, while other critics worry that sex selection will tilt some nations toward military aggression. The human sex ratio varies depending on how one estimates it; there is certainly no one-to-one correspondence between males and females either at birth or across the human (...) lifespan. Complications about who qualifies as ‘male’ and ‘female’ complicate judgments about the ratio even further. Even a judiciously estimated sex ratio does not have, however, the kind of normative status that requires society to refrain from antenatal sex selection. Some societies exhibit lopsided sex ratios as a consequence of social policies and practices, and pragmatic estimates of social needs are a better guide to what the sex ratio should be, as against looking to ‘nature’. The natural sex ratio cannot be a sound moral basis for prohibiting parents from selecting the sex of their children, since it ultimately lacks any normative meaning for social choices. (shrink)

The contribution of women to China’s economic growth and development cannot be overemphasized. Women play important social, economic, and productive roles in any economy. China remains one of the countries in the world with severe gender inequality and sex ratio at birth (SRB) imbalance. Severe gender inequality and disenfranchisement of girls with abnormally high sex ratios at birth reflect deep-rooted sexism and adversely affect girls’ development. For China to achieve economic growth, women should not be ignored and marginalized so (...) that they can contribute to the country’s growth, but the sex ratio at birth needs to be lowered because only women can contribute to growth. Thus, this study empirically predicts an asymmetric relationship between gender inequality, sex ratio at birth and economic growth, using NARDL model over the period 1980–2020. The NARDL results show that increases in gender inequality and sex ratio at birth significantly reduce economic growth in both the short and long term, while reductions in gender inequality and sex ratio at birth significantly boost economic growth in both the short and long term. Moreover, the results show the significant contribution of female labor force participation and female education (secondary and higher education) to economic growth. However, infant mortality rate significantly reduced economic growth. Strategically, the study recommends equal opportunities for women in employment, education, health, economics, and politics to reduce gender disparities and thereby promote sustainable economic growth in China. Moreover, policymakers should introduce new population policy to stabilize the sex ratio at birth, thereby promoting China’s long-term economic growth. (shrink)

Van Lange et al. argue that variations in climate explain cross-societal variations in violence. We suggest that any approach seeking to understand cross-cultural variation in human behavior via an ecological framework must consider a wider array of ecological variables, and we find that income inequality and sex ratio are better predictors than climate of cross-societal variations in violence.

Parental investment theory postulates that where physical condition varies significantly then birth sex ratio will be correlated with social status. Application of this theory to man remains contentious. This study examines physique, wealth, and social status in relation to the sex of live births. It reports a female-biased sex ratio in high social and economic status Nepalese. Close consanguineous marriage, intended to conserve landed wealth within related lineages, and increased female work burdens accompanying larger farm size, are proximate factors which (...) may underlie this finding. A differential payback hypothesis is one way of explaining this pattern. (shrink)

Previous research has suggested that offspring sex ratio may be influenced by the actions of prenatal sex steroids, principally androgens. The relative length of the second (index finger) to the fourth digit (ring finger) has been reported to be a proxy to prenatal testosterone levels. This trait is sexually dimorphic, such that males display a significantly lower 2D:4D ratio (indicating higher testosterone exposure), and this dimorphism appears robust across different populations. We suggest that digit ratio (2D:4D) may form a useful (...) marker to help explain variation in sex ratio and sociosexuality. (shrink)

A study of the abnormal sex ratio at birth in China reveals that it is not an entirely new phenomenon that emerged since the 1980s, but is simply more visible at present. Deliberate intervention to determine the sex of children has existed in the past few decades, at least in certain groups. Apart from modern medical methods, traditional Chinese medical practice is shown to be highly accurate in identifying the sex of a fetus. This may lead to sex-selective abortion and (...) an abnormal sex ratio at birth. The possible causes of the abnormal sex ratio at birth include not only the real imbalance due to the disturbance of social factors, but also a spurious one attributable to the under-counting of female births. The real magnitude of the imbalance has been exaggerated by statistical error. The phenomenon is a complicated one reflecting the comprehensive socioeconomic setting. Among these factors, the stage of the fertility transition is one of the most decisive. (shrink)

Viet Nam has experienced rapid social change over the last decade, with a remarkable decline in fertility to just below replacement level. The combination of fertility decline, son preference, antenatal sex determination using ultrasound and sex selective abortion are key factors driving increased sex ratios at birth in favour of boys in some Asian countries. Whether or not this is taking place in Viet Nam as well is the subject of heightened debate. In this paper, we analyse the nature (...) and determinants of sex ratio at birth in Viet Nam, including a small family size norm, recent reinforcement by the Government of the "one-to-two child" family policy, traditional son preference, easy access to antenatal ultrasound screening and legal abortion, and an increase in the proportion of one-child families. In order to prevent an increased sex ratio at birth in Viet Nam, we argue for the relaxation of the one-to-two child family policy and a return to the policy of "small family size" as determined by families, in tandem with a comprehensive approach to promoting the value of women and girls in society, countering traditional gender roles, and raising public awareness of the negative social consequences of a high sex ratio at birth. (shrink)

In recent years Vietnam has experienced a high sex ratio at birth SRB) amidst rapid socioeconomic and demographic changes. However, little is known about the differentials in SRB between maternal socioeconomic and demographic groups. The paper uses data from the annual Population Change Survey (PCS) in 2006 to examine the relationship of the sex ratio of the most recent birth with maternal socioeconomic and demographic characteristics and the number of previous female births. The SRB of Vietnam was significantly high at (...) 111.4 (95% CI 109.7–113.1) for the period 1st April 2000 to 31st March 2006. Multivariate analysis reveals that sex of the most recent birth is strongly related with the number of previous female births. This association is consistent across different socioeconomic and demographic groups of women. Given the high SRB in Vietnam, further research into the reasons for high SRB in these groups is required, as are intervention programmes such as those raising the public awareness of its negative consequences. © Cambridge University Press, 2010. (shrink)

This study examines the hypothesis that the stopping rule-a traditional postnatal sex selection method where couples decide to cease childbearing once they bear a son-plays a role in high sex ratio of last births (SRLB). The study develops a theoretical framework to demonstrate the operation of the stopping rule in a context of son preference. This framework was used to demonstrate the impact of the stopping rule on the SRLB in Vietnam, using data from the Population Change Survey 2006. The (...) SRLB of Vietnam was high at the level of 130 in the period 1970-2006, and particularly in the period 1986-1995, when sex-selective abortion was not available. Women were 21% more likely to stop childbearing after a male birth compared with a female birth. The SRLB was highest at parity 2 (138.7), particularly in rural areas (153.5), and extremely high (181.9) when the previous birth was female. Given the declining fertility, the stopping rule has a potential synergistic effect with sex-selective abortion to accentuate a trend of one-son families in the population. (shrink)