Women's labor,fertility, and the introduction of modern technology in a rural Maya village

The introduction of mechanized technology into a rural Maya agricultural community in the mid 1970s markedly increased the technology with which maize could be ground and water collected, which in turn introduced a possible savings in the time spent working. This study investigated the response of female fertility to the introduction of this labor-saving technology. Using two proximate determinants of female fertility, the association between the advent of modern technology and changes in the age at which women give birth to their first child and the length of mothers' birth intervals was examined. Analyses showed that women begin their reproductive careers at a younger age after the laborsaving technology was introduced. Estimate of the median age at first birth from the distribution function dropped from 21.2 years before the introduction to 19.5 years after the introduction of the technology. In addition, modeling results show that the probability of a woman giving birth to her first child doubles for any age after the introduction of laborsaving technology. However, changes in birth intervals are less conclusive since the differences of smoothed probability distributions are not significant. Moreover, findings indicate that women who initiate reproduction at a younger age can potentially have longer reproductive careers and larger families.     

The effect of paternal investment on female fertility intention in South Korea

Life history theory views reproduction as an outcome of resource allocation. The allocation of resources such as parental investments of time, energy and material resources involves trade-offs between number of offspring and timing of reproduction. Within the framework of mammalian parental investment, the outstanding feature of human reproduction is the high level of paternal care. Although empirical evidence suggests that human paternal investment may have evolved as a reproductive strategy to reduce infant and child mortality rates, the effects of actual paternal investment, including allocating time to child care, on female reproductive decisions have received relatively little attention. We examined the trade-off from two perspectives using a representative sample of married South Korean women aged 20–44 in 2005 (n=977). First, paternal investment in domestic labor, including child care and housework, was expected to be associated with women's preference regarding future reproduction. Second, relative paternal investment was expected to increase women's preference for future reproduction, especially among employed women. We found that increased paternal investment in child care and housework remarkably enhanced women's intention to have a second child, especially among employed women. In addition, although family members provide a low percentage of child care in South Korea, such help is likely to be a useful resource for second childbirth among employed women. Somewhat expectedly, older age and longer time since first birth had negative effects on women's second-child intention. There is growing evidence that, in the lowest fertility societies, paternal investment may be an essential resource for promoting future reproductive behavior of women, especially employed women.     

Variation in juvenile dependence

Notable in cross-cultural comparisons is the variable span of time between when children become economically self-sufficient and when they initiate their own reproductive careers. That variation is of interest because it shapes the age range of children reliant on others for support and the age range of children available to help out, which in turn affects the competing demands on parents to support multiple dependents of different ages. The age at positive net production is used as a proxy to estimate the close of juvenile economic dependence among a group of Maya subsistence agriculturalists. Maya children produce more than they consume by their early to mid teens but remain in their natal households for a number of years before leaving home and beginning families of their own. The Maya results contrast markedly with those from several groups of hunter-gatherers and horticulturalists for whom we have similar data. Even in the Maya case, where children are self-sufficient at a relatively young age, parents are unable to support their children without help from others. The production surplus of older children appears to help underwrite the cost of large Maya families and subsidize their parents’ continued reproduction.     

Residency Time as an Indicator of Reproductive Restraint in Male Burying Beetles

The cost of reproduction theory posits that there are trade-offs between current and future reproduction because resources that are allocated to current offspring cannot be used for future reproductive opportunities. Two adaptive reproductive strategies have been hypothesized to offset the costs of reproduction and maximize lifetime fitness. The terminal investment hypothesis predicts that as individuals age they will allocate more resources to current reproduction as a response to decreasing residual reproductive value. The reproductive restraint hypotheses predicts that as individuals age they will allocate fewer resources to current reproduction to increase the chance of surviving for an additional reproductive opportunity. In this study, we test for adaptive responses to advancing age in male burying beetles, Nicrophorus orbicollis. Burying beetles use facultative biparental care, but the male typically abandons the brood before the female. Previous work in male burying beetles has suggested several factors to explain variation in male residency time, but no study has observed male behavior throughout their entire reproductive lifetimes to determine whether males change residency time in an adaptive way with age. We compared residency time of males that reproduced biparentally, uniparentally, and on different-sized carcasses to determine if they used an adaptive reproductive strategy. Males did not increase residency time as they aged when reproducing biparentally, but decreased residency time with age when reproducing uniparentally. A decrease in parental care with age is consistent with a reproductive restraint strategy. When female age increased over time, males did not increase their residency time to compensate for deteriorating female condition. To our knowledge, this is the first test of adaptive reproductive allocation strategies in male burying beetles.     

Optimal life‐history schedule in a metapopulation with juvenile dispersal

Previous models have predicted that when mortality increases with age, older individuals should invest more of their resources in reproduction and produce less dispersive offspring, as both their future reproductive value and their prospect of competing with their own sib decline. Those models assumed stable population sizes. We here study for the first time the evolution of age‐specific reproductive effort and of age‐specific offspring dispersal rate in a metapopulation with extinction‐recolonization dynamics and juvenile dispersal. Our model explores the evolutionary consequences of disequilibrium in the age structure of individuals in local populations, generated by disturbances. Life‐history decisions are then shaped both by changes with age in individual performances, and by changes in ecological conditions, as young and old individuals do not live on average in the same environments. Lower juvenile dispersal favours the evolution of higher reproductive effort in young adults in a metapopulation with extinction‐recolonization compared with a well‐mixed population. Contrary to previous predictions for stable structured populations, we find that offspring dispersal should generally increase with maternal age. This is because young individuals, who are overrepresented in recently colonized populations, should allocate more to reproduction and less to dispersal as a strategy to exploit abundant recruitment opportunities in such populations.     

A Novel Quantitative Approach to Women’s Reproductive Strategies

The patterned way in which individuals allocate finite resources to various components of reproduction (e.g. mating effort, reproductive timing and parental investment) is described as a reproductive strategy. As energy is limited, trade-offs between and within aspects of reproductive strategies are expected. The first aim of this study was to derive aspects of reproductive strategies using complete reproductive histories from 718 parous Western Australian women. Factor analysis using a subset of these participants resulted in six factors that represented ‘short-term mating strategy’, ‘early onset of sexual activity’, ‘reproductive output’, ‘timing of childbearing’, ‘breastfeeding’, and ‘child spacing’. This factor structure was internally validated by replication using a second independent subset of the data. The second aim of this study examined trade-offs between aspects of reproductive strategies derived from aim one. Factor scores calculated for each woman were incorporated in generalised linear models and interaction terms were employed to examine the effect of mating behaviour on the relationships between reproductive timing, parental investment and overall reproductive success. Early sexual activity correlates with early reproductive onset for women displaying more long-term mating strategies. Women with more short-term mating strategies exhibit a trade-off between child quantity and child quality not observed in women with a long-term mating strategy. However, women with a short-term mating strategy who delay reproductive timing exhibit levels of parental investment (measured as breastfeeding duration per child) similar to that of women with long-term mating strategies. Reproductive delay has fitness costs (fewer births) for women displaying more short-term mating strategies. We provide empirical evidence that reproductive histories of contemporary women reflect aspects of reproductive strategies, and associations between these strategic elements, as predicted from life history theory.     

The impact of labor-saving technology on first birth intervals in rural Ethiopia

Across the developing world labor-saving technologies introduce considerable savings in the time and energy that women allocate to work. Hormonal studies on natural fertility populations indicate that such a reduction in energetic expenditure (rather than improved nutritional status alone) can lead to increased ovarian function. Other qualitative studies have highlighted a link between labor-saving technology and behavioral changes affecting subsequent age at marriage, which may affect fertility. This biodemographic study was designed to investigate whether these physiological and behavioral changes affect fertility at a population level by focusing on a recent water development scheme in Southern Ethiopia. The demographic consequences of a reduction in women's workload following the installation of water points, specifically the variation in length of first birth interval (time lapsed between marriage and first birth), are investigated. First birth interval length is closely associated with lifetime fertility in populations that do not practice contraception, longer intervals being associated with lower fertility. Using life tables and multivariate hazard modeling techniques a number of significant predictors of first birth interval length are identified. Covariates such as age at marriage, season of marriage, village ecology, and access to improved water supply have significant effects on variation in first birth intervals. When entered into models as a time-varying covariate, access to a water tap stand is associated with an immediate reduction in length of first birth intervals.     

Reproductive investment in pre-industrial humans: the consequences of offspring number, gender and survival

The number and gender of offspring produced in a current reproductive event can affect a mother's future reproductive investment and success. I studied the subsequent reproductive outcome of pre-industrial (1752-1850) Finnish mothers producing twins versus singletons of differing gender. I predicted that giving birth to and raising twins instead of singletons, and males instead of females, would incur a greater reproductive effort and, hence, lead to larger future reproductive costs for mothers. I compared the mothers' likelihood of reproducing again in the future, their time to next reproduction and the gender and survival of their next offspring. I found that mothers who produced twins were more likely to stop breeding or breed unsuccessfully in the future as compared with women of a similar age and reproductive history who produced a same-gender singleton child. As predicted, the survival and gender of the offspring produced modified the costs of reproduction for the mothers. Giving birth to and raising males generally appeared to be the most expensive strategy, but this effect was only detected in mothers who produced twins and, thus, suffering from higher overall costs of reproduction.     

Variation in probability of first reproduction of Weddell seals

1. For many species, when to begin reproduction is an important life-history decision that varies by individual and can have substantial implications for lifetime reproductive success and fitness. 2. We estimated age-specific probabilities of first-time breeding and modelled variation in these rates to determine age at first reproduction and understand why it varies in a population of Weddell seals in Erebus Bay, Antarctica. We used multistate mark-recapture modelling methods and encounter histories of 4965 known-age female seals to test predictions about age-related variation in probability of first reproduction and the effects of annual variation, cohort and population density. 3. Mean age at first reproduction in this southerly located study population (7.62 years of age, SD=1.71) was greater than age at first reproduction for a Weddell seal population at a more northerly and typical latitude for breeding Weddell seals (mean=4-5 years of age). This difference suggests that age at first reproduction may be influenced by whether a population inhabits the core or periphery of its range. 4. Age at first reproduction varied from 4 to 14 years, but there was no age by which all seals recruited to the breeding population, suggesting that individual heterogeneity exists among females in this population. 5. In the best model, the probability of breeding for the first time varied by age and year, and the amount of annual variation varied with age (average variance ratio for age-specific rates=4.3%). 6. Our results affirmed the predictions of life-history theory that age at first reproduction in long-lived mammals will be sensitive to environmental variation. In terms of life-history evolution, this variability suggests that Weddell seals display flexibility in age at first reproduction in order to maximize reproductive output under varying environmental conditions. Future analyses will attempt to test predictions regarding relationships between environmental covariates and annual variation in age at first reproduction and evaluate the relationship between age at first reproduction and lifetime reproductive success.     

Sex,money, and paternity: The evolutionary psychology of domestic violence

Three hundred sixty-five women, with children between six and 12 years of age, were interviewed and tested on various issues theoretically related to domestic violence. The sample was stratified into three subsamples of volunteer women recruited from: (1) a temporary shelter for battered women, (2) the local community and screened specifically for the reported presence of domestic violence, (3) the same community sources and screened only for the reported presence of children of the specified age group. Factor analytic structural equation models were constructed for the predictors of violence by the woman's main sexual partner toward the woman and towards the woman's child. Common factors were constructed for the four major dimensions of domestic violence— verbal, physical, escalated, and sexual—and for the three major predictors of domestic violence—sex, money, and paternity. The sex factor indexed the general quality of the sexual relationship dynamics, the money factor indexed the couple's socioeconomic relations, and the paternity factor indexed the genetic stakes held in the family by the woman's main sexual partner. These three factors jointly accounted for 60% of the variance in violence toward the woman. Violence towards the woman—the only significant direct effect—accounted for 26% of the variance in violence toward the child. These findings suggest that the principal perpetrators of domestic violence may be competitively disadvantaged males, pursuing coercive sexual and parental strategies without regard to the deleterious indirect effects upon their own genetic offspring.     

Family background and female sexual behavior

Since the seminal works of Draper and Harpending (1982) and Belsky et al. (1991) there has been considerable interest in the link between the family environment experienced as a child and consequent mating and reproductive strategy of females. In this paper, predictions from the hypothesis were tested using postal survey data from a cross-section of 415 women in Merseyside, UK. No relationships were found between father-absence, unrelated male-presence, parental divorce or parental death with age at first coitus, number of sexual partners, mean length of sexual relationships or mean length of relationships prior to coitus occurring. This work was supported by an Economic and Social Research Council Studentship. This paper was completed as part of the author’s doctoral research that focused on differences in age at first reproduction between social classes in the UK, conducted at the University of Liverpool. The author now works for the Scottish Executive, Edinburgh.     

Early sexual maturity among Pumé foragers of Venezuela: fitness implications of teen motherhood

Because humans have slow life histories, discussions of the optimal age at first birth have stressed the benefits of delayed reproduction. However, given the diversity of ecological, fertility, and mortality environments in which humans live, reproductive maturity is expected to be highly variable. This article uses reproductive histories to examine a pattern of early menarche and first birth among the Pume, a group of South American foragers. Age at menarche and first birth are constructed using both retrospective and cross‐sectional data for females over the age of 10 (n = 83). The objectives are first to define these patterns and then discuss their reproductive consequences. On average, Pume girls reach menarche at age 12.9, and give birth to their first child at age 15.3–15.5 (retrospective and cross‐sectional data, respectively). This populational average falls several years prior to what often is considered the human norm. Two questions are then considered. What are the infant mortality costs across a mother's reproductive career? How does surviving fertility vary with age at first birth? Results indicate that the youngest of first‐time mothers (<14) are four times more likely to loose their firstborns than older first‐time mothers (≥17). Given parity‐specific mortality rates, the optimal strategy to minimize infant mortality and maximize reproductive span is to initiate childbearing in the midteens. Women gain no additional advantage in surviving fertility by delaying childbearing until their late teens. Am J Phys Anthropol, 2008. © 2008 Wiley‐Liss, Inc.     

The impact of reproduction on gambian women: Does controlling for phenotypic quality reveal costs of reproduction?

Life history theory predicts that where resources are limited, investment in reproduction will cause a decline in body condition and ultimately may lower survival rates. We investigate the relationship between reproduction and mortality in women in rural Gambia. We use a number of different measures of reproductive investment: the timing of reproduction, intensity of reproduction, and cumulative reproductive investment (parity). Though giving birth is clearly a risk factor for increased mortality, we find limited evidence that the timing, intensity, or cumulative effects of reproduction have a survival cost. Instead, there is some evidence that women who have invested heavily in reproduction have higher survival than women with lower reproductive investment: both high parity and late age at last reproduction are associated with high survival. The only evidence for any cost of reproduction is that women who have given birth to twins (considered a marker of heavy investment in reproduction) have higher mortality rates than other women, after the age of 50 years. A potential confounding factor may be differences in health between women: particularly healthy women may be able to invest substantially in both reproduction and their own survival, leading to the positive correlations between survival and both parity and age at last birth we observe. To control for differences in health between women, we reanalyze the relationship between reproduction and mortality but include variables correlating with health in our models (height, BMI, and hemoglobin). Even when controlling for health, the positive correlation between investment in reproduction and survival remains unchanged.     

Parents’ posthumous use of daughter’s ovarian tissue: Ethical dimensions

In recent years, progress in cancer treatment has greatly increased the chances of recovery. Yet, treatment may have irreversible effects on patients’ fertility. In order to protect future fertility, preservation of ovarian tissue may be offered today even to very young girls, involving a surgical procedure that may be performed by minimally invasive laparoscopy, under general anesthesia. However, in the tragic event of a girl’s death, questions may arise regarding the possible use of the preserved ovarian tissue by her parents. Should posthumous reproductive use of ovarian tissue without the girl’s prior consent (due to her young age) be considered a violation of her rights? On the other hand, can it be argued that it is in the interest of a child who died young to leave a genetic trace through posthumous reproduction, because genetic continuity is in the interest of every human being? After presenting the relevant clinical facts, we explore the ethical dimensions of this possible practice through an analysis of the interests of the deceased, her parents, and the child that may be born posthumously.     

Premarital childbearing in northwest Kenya: challenging the concept of illegitimacy

A number of African countries, including Kenya, have experienced a marked rise in births among unmarried women. In Western countries, reproduction outside of marriage is assumed to be illegitimate and a social problem. One hypothesis used to explain the increasing incidence of premarital fertility in Africa is a breakdown of traditional social controls by the extended family over the sexual behavior of adolescents. A competing hypothesis suggests that unmarried women use sexual relations to achieve goals such as marriage. Among Turkana pastoralists of northwest Kenya, we find a pattern of premarital birth that fits either hypothesis only loosely. Premarital fertility among the Turkana is both widespread and culturally acceptable, with over 30 per cent of women having at least one child prior to marriage. Although women with premarital births initiate childbearing on average one year earlier than women with only marital births, women's marital status does not influence the length of the interval between first and second births. Marriage among the Turkana is not a social trigger for the onset and continuation of reproduction or a means to legitimate reproduction. Marital status of the parents simply determines the custody of a child. In a premarital birth, the father pays a set fee to the mother's family, and the custody of the child remains permanently with the mother's family. If the parents later marry, the father must purchase custody of the child by another fee at that time. Since the Turkana have experienced few effects of modernization, the existence of such a practice suggests that cultural factors must be taken into account before assessing premarital fertility in Africa as a social problem.     

Individual heterogeneity in life-history trade-offs with age at first reproduction in capital breeding elephant seals

Recruitment age plays a key role in life-history evolution. Because individuals allocate limited resources among competing life-history functions, theory predicts trade-offs between current reproduction and future growth, survival and/or reproduction. Reproductive costs tend to vary with recruitment age, but may also be overridden by fixed individual differences leading to persistent demographic heterogeneity and positive covariation among demographic traits at the population level. We tested for evidence of intra- and inter-generational trade-offs and individual heterogeneity relating to age at first reproduction using three decades of detailed individual life-history data of 6,439 capital breeding female southern elephant seals. Contrary to the predictions from trade-off hypotheses, we found that recruitment at an early age was associated with higher population level survival and subsequent breeding probabilities. Nonetheless, a survival cost of first reproduction was evident at the population level, as first-time breeders always had lower survival probabilities than prebreeders and experienced breeders of the same age. However, models accounting for hidden persistent demographic heterogeneity revealed that the trade-off between first reproduction and survival was only expressed in “low quality” individuals, comprising 35% of the population. The short-term somatic costs associated with breeding at an early age had no effect on the ability of females to allocate resources to offspring in the next breeding season. Our results provide strong evidence for individual heterogeneity in the life-history trajectories of female elephant seals. By explicitly modeling hidden persistent demographic heterogeneity we show that individual heterogeneity governs the expression of trade-offs with first reproduction in elephant seals.     

Trisomy 21 and maternal age of menopause: does reproductive age rather than chronological age influence risk of nondisjunction?

The biological basis underlying the increased risk of nondisjunction in offspring of women of advanced maternal age is not understood. We sought to test the hypothesis that maternal reproductive age (distance in time from approaching menopause) rather than chronological age is pivotal in the etiology of nondisjunction. Our results found no difference in age of menopause between women 30 years old at delivery of a child with trisomy 21 (i.e., age-related nondisjunction) compared to controls. Among women <30 years of age at delivery of a child with trisomy 21, none underwent premature menopause. Therefore, our findings fail to support the theory that reproductive age plays a major role in the etiology of nondisjunction.     

Survival costs of reproduction are mediated by parasite infection in wild Soay sheep

A trade‐off between current and future fitness potentially explains variation in life‐history strategies. A proposed mechanism behind this is parasite‐mediated reproductive costs: individuals that allocate more resources to reproduction have fewer to allocate to defence against parasites, reducing future fitness. We examined how reproduction influenced faecal egg counts (FEC) of strongyle nematodes using data collected between 1989 and 2008 from a wild population of Soay sheep in the St. Kilda archipelago, Scotland (741 individuals). Increased reproduction was associated with increased FEC during the lambing season: females that gave birth, and particularly those that weaned a lamb, had higher FEC than females that failed to reproduce. Structural equation modelling revealed future reproductive costs: a positive effect of reproduction on spring FEC and a negative effect on summer body weight were negatively associated with overwinter survival. Overall, we provide evidence that parasite resistance and body weight are important mediators of survival costs of reproduction.     

Women's health. The childbearing years and after.

Health and development planners have tended to see women primarily in context of their reproductive role. As a result, solutions to women''s health needs have been restricted to expanding and improving maternal and child health systems. There has recently been a major shift in direction, largely because of the influence of the world conference on population and development held in Cairo in 1994. Dr Guiseppe Benagiano, director of the special programme of research, development and research training in human reproduction based at the WHO, says, "We need to remind ourselves constantly that reproductive health is not simply a biomedical issue but one with serious implications for our general health and by extension, for all our efforts in human social and economic development." The 1993 world development report on health identified the lack of a clear strategy for engaging women in health care and suggested that child health services, prenatal care, treatment of sexually transmitted diseases, and family planning services should be provided jointly at convenient times. In an example of this, the Chilean Institute of Reproductive Medicine now offers integrated family planning services at the same time as child health services, and Thailand is experimenting with mobile health clinics to reach women in their homes. As the proportion of elderly women increases, old age is increasingly being seen as a female issue. With the impact of urbanisation and industrialisation, more of these women are living isolated lives, often suffering from chronic debilitating diseases. In his opening statement to the global commission on women''s health in April 1995 which focused on health conditions of women in old age, Dr Hiroshi Nakajima, the WHO''s director general, said: "Our goal should not be solely to extend lives in the physical sense, but to ensure that the added years are worth living."     

An adaptive model for predicting !Kung reproductive performance: A stochastic dynamic programming approach

A stochastic dynamic programming model is presented that supports and extends work on the reproductive performance of the !Kung Bushmen (Lee 1972; Blurton Jones and Sibly 1978; Blurton Jones 1986), proposing that !Kung women and their reproductive systems may be maximizing reproductive success. The stochastic dynamic programming approach allows the construction of a whole-life model where the physical/environmental constraints along with the uncertainty about future events !Kung women face when making reproductive choices can be explicitly built in. The model makes quantitative predictions for the optimal reproductive strategy assuming !Kung women are maximizing expected lifetime reproduction (ELR) given the physical parameters of !Kung life.The model relies on data gathered from the works cited above and some considerations from simple probability theory. The model predictions for optimal birth spacing match the !Kung reproductive data very well and support earlier findings (Blurton Jones and Sibly; Blurton Jones 1986). The utility of the dynamic modeling approach is illustrated when the effects of varying certain model parameters are investigated.By including the effect of the mother's mortality, which was not included in the Blurton Jones and Sibly (1978) analysis, the model allows for further exploration of the application of an adaptive approach to human reproductive performance. By adding some considerations about the risks of childbirth for the mother the model not only predicts optimal birth spacing, which is site specific, but also predicts the optimal time for a woman to begin and cease having children. These predictions coincide with menarche and menopause and shed light on their possible adaptive value.