Birth spacing and infant and early childhood mortality in a high fertility area of Bangladesh: age-dependent and interactive effects

Childhood mortality and associated factors in Migori County, Kenya: evidence from a cross-sectional survey

OBJECTIVES

The under-five mortality (U5M) rate in Kenya (41 per 1000 live births) remains significantly above international goals (25 per 1000 live births). This is further exacerbated by regional inequalities in mortality. We aimed to describe U5M in Migori County, Kenya, and identify associated factors that can serve as programming targets.

DESIGN

Cross-sectional observational survey.

SETTING

Areas served by the Lwala Community Alliance and control areas in Migori County, Kenya.

PARTICIPANTS

This study included 15 199 children born to respondents during the 18 years preceding the survey.

PRIMARY AND SECONDARY OUTCOME MEASURES

The primary outcome was mortality in the first 5 years of life. The survey was powered to detect a 10% change in various health metrics over time with 80% power.

RESULTS

A total of 15 199 children were included in the primary analyses, and 230 (1.5%) were deceased before the fifth birthday. The U5M rate from 2016 to 2021 was 32.2 per 1000 live births. Factors associated with U5M included year of birth (HR 0.926, p<0.001), female sex (HR 0.702, p=0.01), parental marriage (HR 0.642, p=0.036), multiple gestation pregnancy (HR 2.776, p<0.001), birth spacing less than 18 months (HR 1.894, p=0.005), indoor smoke exposure (HR 1.916, p=0.027) and previous familial contribution to the National Hospital Insurance Fund (HR 0.553, p=0.009). The most common cause of death was malaria.

CONCLUSIONS

We describe factors associated with childhood mortality in a Kenyan community using survival analyses of complete birth histories. Mortality rates will serve as the baseline for future programme evaluation as a part of a 10-year study design. This provides both the hyperlocal information needed to improve programming and generalisable conclusions for other organisations working in similar environments.

Effects of short birth interval on different forms of child mortality in Bangladesh: Application of propensity score matching technique with inverse probability of treatment weighting

BACKGROUND

The prevalence of Short Birth Interval (SBI) is higher in Low- and Middle-Income countries (LMICs), including Bangladesh. Previous studies in LMICs have estimated the effects of SBI on child mortality by comparing two unequal groups of mothers based on their socio-economic status. This approach may lead to overestimation or underestimation of the true effect of birth interval on child mortality, particularly when sample sizes are relatively small.

OBJECTIVE

We determined the effects of SBI on several forms of child mortality in Bangladesh by comparing two equal groups created by applying the propensity score matching technique.

METHODS

This study analyzed data from 5,941 mothers and 1,594 health facilities extracted from the 2017/18 Bangladesh Demographic and Health Survey and the 2017 Bangladesh Health Facility Survey. The exposure variable was SBI (defined as the interval between two subsequent births <33 months: yes, no), while the outcome variables were neonatal mortality (defined as mortality within 28 days of birth: yes, no), infant mortality (defined as mortality within 1 year of birth: yes, no), and under-five mortality (defined as mortality within 5 years of birth: yes, no). Multilevel Poisson regression based on inverse probability treatment weights was used to determine the association between exposure and outcome variables.

RESULTS

The prevalence rates of neonatal, infant, and under-five mortality were 48.8, 30.8, and 23.1 per 1000 live births, respectively. Newborns of SBI mothers were found to have a 63% higher likelihood of neonatal mortality (aPR, 1.63; 95% CI, 1.08-2.46) compared to newborns of non-SBI mothers. Furthermore, the prevalence of infant mortality and under-five mortality was 1.45 times higher (aPR, 1.45; 95% CI, 1.01-2.08) and 2.82 times higher (aPR, 2.82; 95% CI, 2.16-3.70), respectively, among babies born in a short interval of their immediately preceding sibling as compared to babies born in a normal interval of their immediately preceding sibling.

CONCLUSIONS

Findings of this study indicate that SBI is an important predictor of child mortality. Consequently, around 1 million children born in a short interval every year in Bangladesh are at risk of dying before reaching their fifth birthday. This indicates a challenge for Bangladesh to achieve the SDG 3 target to reduce neonatal and under-five mortality to 12 and 25 deaths per 1000 live births, respectively. Hence, awareness-building programs about the adverse effects of SBI and strengthening existing healthcare facilities are important.

Higher offspring mortality with short interbirth intervals in free-ranging rhesus macaques

Short birth intervals have long been linked to adverse child outcomes in humans. However, it remains unclear the extent to which the birth interval has a direct influence on offspring mortality, independent of the confounding effects of modern environments and human sociocultural practices on reproductive behavior. Outside of humans, the relationship between birth intervals and offspring mortality has been rarely tested, leaving an open question of how much the findings from humans imply evolutionarily conserved mechanisms. Here, using ∼9,000 birth records from ∼1,400 free-ranging rhesus macaque mothers, we show that short birth intervals preceding or succeeding the birth of an offspring are both associated with higher offspring mortality, after controlling for heterogeneity across mothers and birth cohorts. We clarify that the mortality risk of a short birth interval to an offspring is contingent on the survival of its older or younger sibling, the condition that reduces maternal resources for investment in the offspring. This finding suggests that life-history tradeoffs between offspring quantity (a short birth interval) and quality (offspring survival) form an evolutionary force shaping variation in birth intervals. Consistent with the well-known observation made in humans, we also found a nonlinear relationship between the preceding interbirth interval and infant mortality. The overall congruence with the findings from the human literature indicates a robust relationship between birth intervals and offspring mortality.

Under-five mortality in the Rongo Sub-County of Migori County, Kenya: Experience of the Lwala Community Alliance 2007-2017 with evidence from a cross-sectional survey

Introduction

Childhood mortality remains a pressing problem in rural Kenya, and reducing under-five deaths is a key target of the Sustainable Development Goals. We aim to describe the reduction in under-five mortality in a rural Kenyan community served by the Lwala Community Alliance and factors associated with under-five mortality in this community.

Methods

A cross-sectional survey containing a complete birth history was administered to a representative sample of the catchment area of the Lwala Community Alliance. Survival analysis techniques were used to describe temporal trends and risk factors related to under-five mortality.

Results

1,362 children were included in the study, and 91 children died before the fifth birthday. The most common causes of death among children under five were malaria (19%), respiratory infection (13%), and anemia (11%). The under-five mortality rate was 104.8 per 1,000 live births from 1999 to 2006 and 53.0 per 1,000 after the founding of the Lwala Community Alliance in 2007. Factors associated with under-five mortality included year of birth (HR 0.931; 95% CI: 0.877, 0.988; p = 0.019), multiple-gestation pregnancy (HR 6.201; 95% CI: 2.073, 18.555; p < 0.001), and birth in the long rain season (HR 1.981; 95% CI: 1.350, 2.907; p < 0.001). Birth spacing greater than 18 months was negatively associated with under-five mortality (HR 0.345; 95% CI: 0.203, 0.587; p < 0.001).

Conclusions

There was a significant decrease in under-five mortality before and after the presence of the Lwala Community Alliance. Multiple-gestation pregnancies, birth season, and short birth spacing were associated with under-five mortality and provide possible targets to further reduce mortality in the region. This provides both hyper-local data necessary for implementation efforts and generalizable data and sampling methods that may be useful for other implementing organizations in sub-Saharan Africa.

Early-life mortality clustering in families: A literature review

Research on early-life mortality in contemporary and historical populations has shown that infant and child mortality tend to cluster in a limited number of high-mortality families, a phenomenon known as 'mortality clustering'. This paper is the first to review the literature on the role of the family in early-life mortality. Contemporary results, methodological and theoretical shortfalls, recent developments, and opportunities for future research are all discussed in this review. Four methodological approaches are distinguished: those based on sibling deaths, mother heterogeneity, thresholds, and excess deaths in populations. It has become clear from research to date that the death of an older child harms the survival chances of younger children in that family, and that fertility behaviour, earlier stillbirths, remarriages, and socio-economic status all explain mortality clustering to some extent.

Maternal high-risk fertility behavior and association with chronic undernutrition among children under age 5 y in India, Bangladesh, and Nepal: Do poor children have a higher risk?

OBJECTIVES

We aimed to examine whether an association exists between maternal high-risk fertility behavior and chronic undernutrition among children under 5 y of age. In addition, we explored the relationship between poverty and high-risk fertility behavior and the relative roles they play as obstacles in the reduction of the risk of undernutrition among children.

METHODS

The analysis was based on responses from married women ages 15 to 49 who lived with at least one child under the age of 5; and three cross-sectional, nationally representative samples from India, Bangladesh, and Nepal were considered.

RESULTS

Maternal high-risk fertility behavior was associated with an increased risk of chronic undernutrition among children in India, Bangladesh, and Nepal. Multiple high-risk categories appeared to have more profound consequences on the outcomes measured. Findings also demonstrated that with regard to the risk of undernutrition, children of mothers who were either poor or who experienced high-risk fertility were not uniquely disadvantaged.

CONCLUSIONS

The results suggest that with regard to the risk of chronic undernutrition, the negative effect of high-risk fertility behavior extends across all economic backgrounds and is not limited to children of mothers who were either poor or who experienced high-risk fertility.

Reduced birth intervals following the birth of children with long-term illness: evidence supporting a conditional evolved response

Human birth interval length is indicative of the level of parental investment that a child will receive: a short interval following birth means that parental resources must be split with a younger sibling during a period when the older sibling remains highly dependent on their parents. From a life-history theoretical perspective, it is likely that there are evolved mechanisms that serve to maximize fitness depending on context. One context that would be expected to result in short birth intervals, and lowered parental investment, is after a child with low expected fitness is born. Here, data drawn from a longitudinal British birth cohort study were used to test whether birth intervals were shorter following the birth of a child with a long-term health problem. Data on the timing of 4543 births were analysed using discrete-time event history analysis. The results were consistent with the hypothesis: birth intervals were shorter following the birth of a child diagnosed by a medical professional with a severe but non-fatal medical condition. Covariates in the analysis were also significantly associated with birth interval length: births of twins or multiple births, and relationship break-up were associated with significantly longer birth intervals.

Girl helpers and time allocation of nursing women among the Toba of Argentina

In this paper we outline the activities of young girls in a Toba community of northern Argentina and examine the effect of girl helpers on time allocation of nursing women. Activity budgets were obtained for 41 girls aged 3 to 15 using spot observations. Girls spent substantial portions of observations engaged in helping behaviors. Individual values varied with age, anthropometric characteristics, and birth order. Activity budgets of 21 nursing women were obtained through focal observation sessions. Women living in households with girls aged 7 to 15 allocated 17% less time to domestic work and 9% more time to socializing during afternoon observation sessions. For nursing women in this community, direct childcare (provided by the infant's own mother) seemed to be a priority. Living with a girl helper did not have any measurable effect on the frequency or duration of nursing, or on the time that women spent caring for their infants. Based on these findings, hypotheses are outlined for future work on the effect of girl helpers on women's fertility.

The effect of maternal age and reproductive history on offspring survival and lifetime reproduction in preindustrial humans

Senescence is one of the least understood aspects of organism life history. In part, this stems from the relatively late advent of complete individual-level datasets and appropriate statistical tools. In addition, selection against senescence should depend on the contribution to population growth arising from physiological investment in offspring at given ages, but offspring are rarely tracked over their entire lives. Here, we use a multigenerational dataset of preindustrial (1732-1860) Finns to describe the association of maternal age at offspring birth with offspring survival and lifetime reproduction. We then conduct longitudinal analyses to understand the drivers of this association. At the population level, offspring lifetime reproductive success (LRS) declined by 22% and individual λ, which falls with delays to reproduction, declined by 45% as maternal age at offspring birth increased from 16 to 50 years. These results were mediated by within-mother declines in offspring survival and lifetime reproduction. We also found evidence for modifying effects of offspring sex and maternal socioeconomic status. We suggest that our results emerge from the interaction of physiological with social drivers of offspring LRS, which further weakens selection on late-age reproduction and potentially molds the rate of senescence in humans.

Effects of birth spacing on maternal, perinatal, infant, and child health: a systematic review of causal mechanisms

This systematic review of 58 observational studies identified hypothetical causal mechanisms explaining the effects of short and long intervals between pregnancies on maternal, perinatal, infant, and child health, and critically examined the scientific evidence for each causal mechanism hypothesized. The following hypothetical causal mechanisms for explaining the association between short intervals and adverse outcomes were identified: maternal nutritional depletion, folate depletion, cervical insufficiency, vertical transmission of infections, suboptimal lactation related to breastfeeding-pregnancy overlap, sibling competition, transmission of infectious diseases among siblings, incomplete healing of uterine scar from previous cesarean delivery, and abnormal remodeling of endometrial blood vessels. Women's physiological regression is the only hypothetical causal mechanism that has been proposed to explain the association between long intervals and adverse outcomes. We found growing evidence supporting most of these hypotheses.

Is sibling rivalry fatal?: siblings and mortality clustering

Evidence drawn from nineteenth-century Belgian population registers shows that the presence of similarly aged siblings competing for resources within a household increases the probability of death for children younger than five, even when controlling for the preceding birth interval and multiple births. Furthermore, in this period of Belgian history, such mortality tended to cluster in certain families. The findings suggest the importance of segmenting the mortality of siblings younger than five by age group, of considering the presence of siblings as a time-varying covariate, and of factoring mortality clustering into analyses.

Infant death clustering in families: magnitude, causes, and the influence of better health services, Bangladesh 1982-2005

This analysis of infant mortality in Bangladesh focuses on explaining death clustering within families, using prospective data from a rural region in Bangladesh, split into areas with and without extensive health services (the area covered by the International Centre for Diarrhoeal Disease Research and the comparison area, respectively). The modelling framework distinguishes between two explanations of death clustering: (observed and unobserved) heterogeneity across families and a causal 'scarring' effect of the death of one infant on the survival chances of the next to be born. Keeping observed and unobserved characteristics constant, we find scarring in the comparison area only. There the likelihood of infant death is about 29 per cent greater if the previous sibling died in infancy than otherwise. This effect mainly works through birth intervals: infant deaths are followed by shorter birth intervals, which increases the risk of infant death for the next child.

Systematic review of birth cohort studies in South East Asia and Eastern Mediterranean regions

BACKGROUND

Few longitudinal studies of children have taken place in the developing world, despite child mortality being concentrated there. This review summarises the methodologies and main outcomes of longitudinal studies of pre-school children (0 to 59 months) in the World Health Organization's South East Asia (SEA) and Eastern Mediterranean (EM) Regions.

METHODS

A systematic search of literature using pre-defined criteria revealed 7863 papers. After application of quality criteria, 120 studies were selected for analysis.

RESULTS

The search revealed 83 studies in the SEA region and 37 in the EM region, of which 92 were community-based and 8 facility-based. Objectives were diverse but topics included growth (n = 49 studies), mortality (n = 28), nutrition (n = 24), and infectious diseases (n = 33). Only 12 studies focused on non-communicable diseases. Duration ranged from 7 to 384 months. Measurements included anthropometric (n = 56 studies), socioeconomic (n = 50) and biological sampling (n = 25), but only one study was DNA-based.

CONCLUSION

Biobanks have emerged as the most successful approach to generating knowledge about disease causes and mechanisms. Little of this is possible to undertake in the in SEA or EM regions, however. Further longitudinal studies of young children with DNA sampling should be set up to better understand determinants of diseases in low-income countries.

Intra- and inter-household differences in antenatal care, delivery practices and postnatal care between last neonatal deaths and last surviving children in a peri-urban area of India

Nearly a quarter of the world's neonatal deaths take place in India. The state of Uttar Pradesh alone accounts for one-quarter of all neonatal deaths in the country. In this study 892 married women aged less than 50 years living in a peri-urban area of Kanpur city in Uttar Pradesh were interviewed. In all, 109 women reported neonatal deaths. Characteristics of the last neonatal deaths of these 109 women were compared with those of the last surviving children. Also, characteristics of women who had a neonatal death were compared with those of 783 women who had no neonatal death. It was found that as compared with neonatal deaths, the last surviving children of the 109 women had: (a) significantly better antenatal tests during pregnancy, intake of iron/folic acid tablets and higher percentage of tetanus toxoid immunization; (b) safer delivery practices such as a higher percentage of institutional delivery, sterilization of instruments and application of antiseptic after removal of umbilical cord; (c) postnatal care, such as application of antiseptic to the navel and postnatal checkups; and (d) higher maternal age and greater birth spacing. Likewise, better antenatal care and safer delivery practices and postnatal care were observed among the 783 women with no neonatal deaths, when compared with women who had experienced neonatal death. The complexities of inter- and intra-household differences in health care are discussed. The paper concludes that to improve child survival general education and awareness regarding safe delivery should be increased. Continuing cultural stigmas and misconceptions about birth practices before, during and after childbirth should be an important part of the awareness campaigns.

Polygyny and women's health in sub-Saharan Africa

In this paper we review the literature on the association between polygyny and women's health in sub-Saharan Africa. We argue that polygyny is an example of "co-operative conflict" within households, with likely implications for the vulnerability of polygynous women to illness, and for their access to treatment. We begin with a review of polygyny and then examine vulnerability to sexually transmitted infections (STIs, including HIV) and differential reproductive outcomes. Polygyny is associated with an accelerated transmission of STIs, both because it permits a multiplication of sexual partners and because it correlates with low rates of condom use, poor communication between spouses, and age and power imbalances among other factors. Female fertility is affected by the interplay between marital rank, household status, and cultural norms in polygynous marriages. Finally, we present areas which have received only cursory attention: mental health and a premature, "social" menopause. Although data are scarce, polygyny seems to be associated with higher levels of anxiety and depression, particularly around stressful life events. It is our hope that the examples reviewed here will help build a framework for mixed method quality research, which in turn can inform decision makers on more appropriate, context-dependent health policies.

The epidemiology of good nutritional status among children from a population with a high prevalence of malnutrition

Objectives:

To identify socio-economic demographic and environmental factors that predict better height-for-age for children under 5 years of age in a Dhaka slum population.

Design:

A panel survey, conducted between 1995 and 1997. A random sample of households was selected. Socio-economic, demographic and environmental variables were collected monthly by questionnaire and nutritional status was assessed.

Setting:

Dhaka slums in Bangladesh.

Subjects:

Three hundred and ninety-two children, surveyed in September–November 1996.

Main outcome measures:

Height-for-age Z-score (HAZ) above −2.

Results:

Thirty-one per cent of children had HAZ <−2. Logistic regression adjusted for cluster sampling showed that better nourished children were more likely to have taller mothers, to be from female-headed households and from families with higher income, electricity in the home, better latrines, more floor space and living in Central Mohammadpur. Better nourished children were less likely to have fathers who have taken days off from work due to illness.

Conclusions:

Interest in ‘positive deviance’ is motivated by the recognition that a focus on the malnourished only – the bottom tail of the distribution – provides indications of how families fail, but not of how they succeed in maintaining child nutrition in the face of adversity. Our analysis provides an alternative perspective on nutrition and vulnerability in an urban slum setting.

An energy-saving development initiative increases birth rate and childhood malnutrition in rural Ethiopia

BACKGROUND

Evolutionary life history theory predicts that, in the absence of contraception, any enhancement of maternal condition can increase human fertility. Energetic trade-offs are likely to be resolved in favour of maximizing reproductive success rather than health or longevity. Here we find support for the hypothesis that development initiatives designed to improve maternal and child welfare may also incur costs associated with increased family sizes if they do not include a family planning component.

METHODS AND FINDINGS

Demographic and anthropometric data were collected in a rural Ethiopian community benefiting from a recent labour-saving development technology that reduces women's energetic expenditure (n = 1,976 households). Using logistic hazards models and general linear modelling techniques, we found that whilst infant mortality has declined, the birth rate has increased, causing greater scarcity of resources within households.

CONCLUSIONS

This study is, to our knowledge, the first to demonstrate a link between a technological development intervention and an increase in both birth rate and childhood malnutrition. Women's nutritional status was not improved by the energy-saving technology, because energy was diverted into higher birth rates. We argue that the contribution of biological processes to increased birth rates in areas of the developing world without access to modern contraception has been overlooked. This highlights the continued need for development programmes to be multisectoral, including access to and promotion of contraception.

Effects of preceding birth intervals on neonatal, infant and under-five years mortality and nutritional status in developing countries: evidence from the demographic and health surveys

OBJECTIVE

This paper examines the association between birth intervals and infant and child mortality and nutritional status.

METHODS

Repeated analysis of retrospective survey data from the Demographic and Health Surveys (DHS) program from 17 developing countries collected between 1990 and 1997 were used to examine these relationships. The key independent variable is the length of the preceding birth interval measured as the number of months between the birth of the child under study (index child) and the immediately preceding birth to the mother, if any. Both bivariate and multivariate designs were employed. Several child and mother-specific variables were used in the multivariate analyses in order to control for potential bias from confounding factors. Adjusted odds ratios were calculated to estimate relative risk.

RESULTS

For neonatal mortality and infant mortality, the risk of dying decreases with increasing birth interval lengths up to 36 months, at which point the risk plateaus. For child mortality, the analysis indicates that the longer the birth interval, the lower the risk, even for intervals of 48 months or more. The relationship between chronic malnutrition and birth spacing is statistically significant in 6 of the 14 surveys with anthropometric data and between general malnutrition and birth spacing in 5 surveys. However, there is a clear pattern of increasing chronic and general undernutrition as the birth interval is shorter, as indicated by the averages of the adjusted odds ratios for all 14 countries.

CONCLUSION

Considering both the increased risk of mortality and undernutrition for a birth earlier than 36 months and the great number of births that occur with such short intervals, the author recommends that mothers space births at least 36 months. However, the tendency for increased risk of neonatal mortality for births with intervals of 60 or more months leads the author to conclude that the optimal birth interval is between 36 and 59 months. This information can be used by health care providers to counsel women on the benefits of birth spacing.

The pattern and factors associated with child spacing in eastern Saudi Arabia

The aim of this cross-sectional study was to determine the patterns of child spacing/birth intervals in the Eastern Province of Saudi Arabia and the socio-economic factors related to them. The study sample comprised nine to 10-year-old Saudi school children randomly selected from 14 schools in Khobar, Thoghba and Dhahran in the Eastern Province of Saudi Arabia. A two-stage sampling technique was applied and yielded a sample of 536 children. The SPSS programme was used for data analysis. The mean preceding and succeeding birth intervals of children in the study were 26.2 +/- 13.7 and 28.2 +/- 12 months respectively. The most important variables that were found to be significantly correlated with the birth intervals were maternal age, level of education, family size and breastfeeding. This is the first time birth intervals were studied for an urban area in Saudi Arabia and the Gulf region. The figures-obtained are essential for any planning of mother and child health interventions. Health team members need to know this information and the variables related to them in order to advise mothers. The figures are also essential in planning measures to reduce infant and child morbidity and mortality.

Birth spacing, sibling rivalry and child mortality in India

The detrimental impact of short preceding birth intervals on infant and early childhood mortality is well documented in demographic literature, although the pathways of influence within the relationship remain an area of debate. This paper examines the impact of the length of the preceding birth interval on under-two mortality in India, and examines the pathways through which short preceding birth intervals may lead to an increased risk of mortality. Three mortality periods are examined: neonatal, early post neonatal and late post-neonatal and toddler, using the 1992 Indian National Family Health Survey. A multilevel modelling approach is used to account for the hierarchical nature of the data. The determinants of infants following a short or long birth interval are also examined. The results show that short preceding birth intervals (< 18 months) are associated with an increased risk of mortality in all three age groups, and the effect is particularly marked in the early post-neonatal period. Significant interactions were found between the length of the preceding birth interval and maternal education, gender and the survival status of the previous child. The significance of these interactions varied with the age of the child. The results highlight the diluting effect that a higher level of maternal education has on the relationship between short preceding birth intervals and mortality risk. There is evidence to suggest that sibling rivalry is a pathway through which short birth intervals influence mortality, with the death of the previous sibling removing the competition for scarce resources, and resulting in lower risks of mortality than if the previous sibling was still alive. The greatest risks of an infant following a short birth interval are among those whose previous sibling died, high parities, those with young mothers, and those whose previous sibling was breastfed for a short duration.

The effects of kin on child mortality in rural Gambia

We analyzed data that were collected continuously between 1950 and 1974 from a rural area of the Gambia to determine the effects of kin on child mortality. Multilevel event-history models were used to demonstrate that having a living mother, maternal grandmother, or elder sisters had a significant positive effect on the survival probabilities of children, whereas having a living father, paternal grandmother, grandfather, or elder brothers had no effect. The mother's remarriage to a new husband had a detrimental effect on child survival, but there was little difference in the mortality rates of children who were born to monogamous or polygynous fathers. The implications of these results for understanding the evolution of human life-history are discussed.

Waiting for Trivers and Willard: do the rich really favor sons?

Parental investment theory has been put forward as a major evolutionary argument explaining male or female biased birth sex ratio, the Trivers-Willard (T-W) hypothesis, predicting that parents living in good circumstances will bias their investment to sons, whereas parents in poor circumstances will bias their investment toward daughters. Tests of the T-W hypothesis on human beings have shown limited evidence for parents appearing to differentiate their investment to sons or daughters according to the reproductive potential of each sex. The present study tests the T-W hypothesis among a large contemporary Polish sample using first birth interval and extent of breastfeeding as measures of parental investment, and economic status and level of parental education as measures of parental condition. The extents to which parental investment and markers of parental condition vary by sex of the child were examined using log-linear analysis. Weak support for the T-W effect is found among families where fathers were best educated, where a greater proportion of first-born boys are breastfed longer than girls, while the opposite trend is observed among families with fathers with lowest levels of education. Although the present study does not fully support the T-W hypothesis, it gives evidence of greater investment in female offspring at the lower extremes of income, and greater investment in males at higher levels of income.

Evolutionary ecology of human life history

The human life history is characterized by several unusual features, including large babies, late puberty and menopause, and the fact that there is a strong cultural influence on reproductive decisions throughout life. In this review I examine human life history from an evolutionary ecological perspective. I first review the evidence for life history trade-offs between fertility and mortality in humans. Patterns of growth, fertility and mortality across the life span are then discussed and illustrated with data from a traditional Gambian population. After outlining the stages of the human life course, I discuss two phenomena of particular interest in evolutionary anthropology, both of which are apparently unique to humans and neither yet fully understood. First, I discuss the evolution of menopause, the curtailing of female reproduction long before death. The evidence that this evolved because investment in existing children's future reproductive success is more important than continuing child bearing into old age is reviewed, along with data relating to the biological constraints that may be operating. Second, I discuss the demographic transition. Declining fertility at a time of increasingly abundant resources represents a serious challenge to an evolutionary view of human life history and behaviour, and is thus examined in detail. Parental investment in children in competition with each other may be key to understanding both of these unusual human phenomena. Copyright 2000 The Association for the Study of Animal Behaviour.

Repeated neonatal deaths in families with special reference to causes of death

It is recognized that one infant death in a family indicates an increased risk of death of a subsequent sibling. This study examines which cause of death of a sibling is related to the mortality of the younger sibling and when. Longitudinal vital events data from the maternal and child health and family planning (MCH-FP) project and the comparison areas in Matlab, Bangladesh, were used. Primary causes of 868 neonatal deaths and 624 post-neonatal deaths resulting from 18,865 singleton live births in 1989-92 and those (967 as neonates and 708 as post-neonates) of their immediate elder siblings were categorised into infectious and non-infectious diseases. Multinomial logistic regression was used to estimate the risk of younger siblings dying in each age period from infectious and non-infectious diseases given the age and cause of deaths of older siblings and controlling for other biosocial correlates of infant mortality. A neonatal death of non-infectious causes in a family was twice as likely to be followed by another one occurring at the same age from similar causes compared with a surviving infant followed by a neonatal death from non-infectious causes. The MCH-FP project, though successful in reducing the risk of neonatal and post-neonatal mortality from infectious diseases, did not reduce the risk of dying from non-infectious diseases.

The coevolution of human fertility and wealth inheritance strategies

Life history theory concerns the scheduling of births and the level of parental investment in each offspring. In most human societies the inheritance of wealth is an important part of parental investment. Patterns of wealth inheritance and other reproductive decisions, such as family size, would be expected to influence each other. Here I present an adaptive model of human reproductive decision-making, using a state-dependent dynamic model. Two decisions made by parents are considered: when to have another baby, and thus the pattern of reproduction through life; and how to allocate resources between children at the end of the parents' life. Optimal decision rules are those that maximize the number of grandchildren. Decisions are assumed to depend on the state of the parent, which is described at any time by two variables: number of living sons, and wealth. The dynamics of the model are based on a traditional African pastoralist system, but it is general enough to approximate to any means of subsistence where an increase in the amount of wealth owned increases the capacity for future production of resources. The model is used to show that, in the unpredictable environment of a traditional pastoralist society, high fertility and a biasing of wealth inheritance to a small number of children are frequently optimal. Most such societies are now undergoing a transition to lower fertility, known as the demographic transition. The effects on fertility and wealth inheritance strategies of reducing mortality risks, reducing the unpredictability of the environment and increasing the costs of raising children are explored. Reducing mortality has little effect on completed family sizes of living children or on the wealth they inherit. Increasing the costs of raising children decreases optimal fertility and increases the inheritance left to each child at each level of wealth, and has the potential to reduce fertility to very low levels. The results offer an explanation for why wealthy families are frequently also those with the smallest number of children in heterogeneous, post-transition societies.