Strand Organiser: Rebecca Sear, London School of Economics
Helpful grandmothers in rural Ethiopia: A study of the role of kin in determining child survival and growth.
Mhairi A. Gibson & Ruth Mace
University College London
Evolutionary kin selection and parental investment theories state that individuals can increase their inclusive fitness (genetic representation in future generations) by extending support to their relatives; however not all kin are equally helped or helpful. Here we document differential patterns of kin support in a rural Ethiopian community where post-marital residence patterns provide differential access to relatives.
Using demographic and anthropometric data collected from rural villages in Southern Ethiopia we are able to identify effects of presence of kin on child mortality and growth patterns. Multivariate event history analyses and general linear models are used to control for the effects of age, sex, birth order, family size and socio-economic status. Additionally, time allocation data provide detailed information on the role of kin within the household.
Mortality analyses indicate that presence of grandmothers had a positive effect on child survival. Maternal grandmothers had a particularly beneficial effect on child height, but paternal grandmothers less so. Time allocation data suggest that grandmothers preferentially visited their daughters' households, irrespective of proximity and, when present, assisted their daughters in heavy domestic tasks rather than helping directly with grandchild care. Matrilocal post-marital residence was associated with improved child survival, but lower levels of child nutritional status (low height and low weight for height). The negative effect of matrilocal residence on child nutritional status is interesting, since it contradicts the other positive findings of maternal kin. This may be due to wealth effects, increased competition between siblings or higher survival of smaller infants in matrilocal households.
Department of Anthropology, University College London, Gower Street, London, UK.
Email: mhairi.gibson@ucl.ac.uk
Maternal care and child's survival in humans: an evolutionary demography perspective
Samuel Pavard1 and Evelyne Heyer2
Max Planck Institute for Demographic Research & Musée de l'Homme
Introduction - Evolutionary demography provides evidence that offspring's survival until maturity is a major determinant of age trajectories of females' survival and fertility. These studies usually assume that offspring survival until maturity is independent of mothers' survival. However, in humans, children's survival depends mainly on maternal care. Therefore we expect a strong relationship between mother's and child's survival in our species. In the present study, we investigate whether an increase of this latter relationship during human evolution (i.e. an increase of maternal care involvement in child's survival) can explain observed human vital rates.
Methods - We constructed two evolutionary scenarios modelling an increase of maternal care. The first leads to an increase in children's mean survival until maturity. The second leads to a decrease in children's survival because of a higher level of child's altriciality (children's survival is more dependent of maternal care). Perturbation analyses of the Finite Rate of Increase 1 were performed in both cases.
Results - Both scenarios lead to an increase in lifespan (even after menopause), to a higher fertility at the peak of reproductive life and to a decrease in fertility at the end of the reproductive life. These features are the main characteristic of human life history. Furthermore, our model allows associations between features not expected in models assuming that children's survival is independent of mother's survival: an increase in lifespan with an increase in mean children's survival or an increase in fertility.
1 Laboratory of Survival and Longevity, Max Planck Institute for Demographic Research, Rostock, Germany.
2 Département Hommes, Nature et Sociétés, CNRS UMR 5145/Musée de l'Homme, Paris, France.
Email : pavard@demogr.mpg.de
Reproductive effort, immunosenescence and longevity in pre-industrial Finns
Virpi Lummaa1, Samuli Helle2 and Jukka Jokela2
1University of Sheffield, 2University of Turku
The trade-off between reproduction and longevity has been proposed to be mediated by the reproductive costs accelerating senescence of the immune system, leading to shorter lifespan. These physiological costs should be
buffered by the resources available for the mothers during pregnancy and lactation so that costs should be lower for mothers in good condition. We used individual-based historical population records maintained by the Lutheran
church to examine how increased reproductive effort (delivery of twins vs. singletons and physiologically more expensive sons vs. daughters) affected survival and the probability of dying from an infectious disease among women
and men living in pre-industrial Finland between 1700-1860. First, we found that mothers delivering twins had reduced post reproductive survival. This effect arose because mothers of twins had a higher probability of succumbing to an infectious disease (mainly tuberculosis) than mothers delivering singletons only. The risk was further elevated if these mothers had started their reproduction young. Second, we found evidence that production of
physiologically more costly sons shortened maternal lifespan. The magnitude of this effect corresponds to a 34-week reduction in longevity per each son delivered. The number of delivered sons increased the probability of dying of
an infectious disease among women living in resource-limited conditions (low social class), but not in higher social classes. In contrast, longevity and disease mortality of men were independent of reproductive effort, suggesting
that females carried higher costs of reproduction. Our results suggest that increased physiological cost of reproduction promoted immunosenescence in mothers living under limited resources.
1University of Sheffield
2 University of Turku, Finland
Email: v.lummaa@sheffield.ac.uk
Intergenerational transfers in humans and other animals
Jan Beise
Max Planck Institute for Demographic Research
The increasing ageing of many societies has brought the issue of intergenerational transfers back in the centre of interests not only for demographers and social scientists but also for researchers interested in evolutionary demography. Recent studies of current traditional populations and of historic populations have indicated that transfers running down the generations have substantial impact on survival and/or fertility of children. It could be shown that the provider of crucial transfers are not only the parents but also other relatives and in particular the grandparents. These findings have stimulated the research of intergenerational transfers in a wider evolutionary context: Transfers can be found not only in humans but also in animals (where it is usually studied under the label "parental care"). In fact, if intergenerational transfers are defined loosely as resources passed between generations to the benefits of the recipients virtually all sexually reproducing organisms show some transfers since even the provisioning of egg cells with nutritional material can then be regarded as transfers.
The purpose of this paper is to give an overview of intergenerational transfers in human and other animals and discuss its implications for life history. Of particular interest here is to show that transfers are flowing during all developmental stages, from the time around gamete production up to transfers received in fully adult and independent ages. Furthermore, transfers can have all kind of currencies; they can be given in the form of material resources but also as allocation of time and as the provision of protection, support or knowledge. Transfers are an important life history trait and the extent of transfers provided seems to be interrelated with other traits like longevity and social complexity.
Dr Jan Beise, Max Planck Institute for Demographic Research, Konrad-Zuse-Str. 1,
18057 Rostock, Germany
Email: janbeise@yahoo.com
Causes and consequences of parental divorce in relation to offspring gender: an evolutionary perspective
David W. Lawson
University College London
It is the aim of this paper to explore the usefulness of an evolutionary perspective in understanding the causes of parental marital disruption (PMD) and its consequences in general, and in relation to the factor of offspring gender in particular. A Darwinian model of marriage as formalised pair bonding and its relation to sex-biased parental investment is introduced and shown to be consistent with the literature. In particular, recent national level US studies demonstrating that daughter-biased families are more likely to experience parental divorce are discussed as potential evidence of sex-biased parental investment in westernised society. Predictions are then generated, and tested using the 1958 National Child Development Study (n: males=3411, females=3767), regarding the effect of PMD on the reproductive behaviour of British adults: 1) PMD is predicted to be associated with a relatively "fast and early" strategy of reproduction, 2) parental divorce and parental death are predicted to have equivalent effects on reproductive behaviour, 3) PMD is predicted to have the largest effects on the reproductive behaviour of those who were the youngest age at the time of PMD event, and finally, and of principal focus 4) PMD is predicted to have the largest effects on the reproductive behaviour of sons rather than daughters. Predictions 1 and 2 receive solid support, however predictions 3 and 4 are not met by the data. While the findings regarding the timing of PMD are confusing, the findings regarding offspring gender may be understood by recognising problematic assumptions made by this prediction in particular
David W. Lawson, Department of Anthropology, University College London
Gower Street, London WC1E 6BT
Email: dav_lawson@yahoo.co.uk
Is the gender gap in religiosity a legacy of evolution?
David Voas
University of Manchester
Women appear to be more religious than men in most places for which we have evidence. The existence and universality of this 'gender gap' prompt a number of questions. Does its size vary by country, religion or degree of modernization, as would be expected if it is the product of social forces? Or is it relatively invariant over time and across societies, thus suggesting that it is a natural phenomenon? The European Social Survey, a new programme covering more than 20 nations, is a good place to look for answers. A religiosity scale can be constructed using variables for religious affiliation, frequency of attendance and prayer, self-description as religious (or not), and importance of religion in life. Additional data are available from the British Social Attitudes surveys and the last population census. Although the gender gap has narrowed a little in some countries as overall religious involvement has declined, the findings broadly support the idea that women are naturally more religious than men. Is it possible to construct a good evolutionary theory that explains the association of femininity and religiosity?
Cathie Marsh Centre for Census and Survey Research, Crawford House
University of Manchester, Manchester M13 9PL
Email: voas@man.ac.uk
Rebecca Sear
London School of Economics
Relationships between adult mortality and anthropometric status (such as height and weight) have received considerable attention in developed countries, where the causes of death are the chronic diseases that affect well-nourished and medically well cared for populations. Less is known about these relationships in the developing world, where acute diseases are more important causes of death. This paper uses a life history framework to examine the relationship between anthropometric status and mortality in a rural Gambian population, using height, weight and haemoglobin as the measures of interest. A simple reading of life history theory would lead to the expectation that individuals will use their available energy to lower mortality rates, so that anthropometric measures (which provide an indication of energy availability) should correlate negatively with mortality risk. This study finds that only for haemoglobin is there evidence for a linear negative relationship between the anthropometric variable and mortality risk for both sexes: individuals with higher haemoglobin have lower mortality risk. BMI is broadly negatively related to mortality risk for both men and women, though overweight individuals do have a slightly increased risk of death. The relationship between height and mortality differs between the sexes. For men, there is no relationship between height and mortality. For women there is a U-shaped relationship, so that both short and tall women have higher mortality risks than women of average height.
Department of Social Policy, London School of Economics, Houghton St, London WC2A 2AE. Tel: 020 7955 7348
Email: r.sear@lse.ac.uk
Understanding why teenage motherhood occurs in deprived environments: an evolutionary approach
Sarah E. Johns
University of Kent at Canterbury
The link between teenage motherhood and dangerous, deprived environments is relatively well established, with neighbourhood-wide measures of socio-economic status used to measure levels of deprivation. However, convincing explanations of why such a link should exist are lacking. Using life-history theory, this study explores the hypothesis that teenage motherhood is the result of an evolved reproductive strategy that allows for variation in life-history event timings, and that having children at an earlier age may promote lineage survival when the environment is unstable and risky, and where personal future is uncertain. This hypothesis predicts that women's reproductive decisions should be influenced by their experience of the nature of their environment. This may not be accurately reflected in neighbourhood-wide measures, but relatively few studies have examined individuals' subjective perceptions of environmental conditions. In this study, two measures of environmental conditions are evaluated: (1) a neighbourhood measure of socio-economic status (average weekly household income for the ward of residence), and (2) women's subject perception of their environment (perceived environmental risk).
Data were collected by postal questionnaire from a large sample of women living in the county of Gloucestershire, UK, and results indicate that a woman's perception of her environment may be a better predictor of teenage motherhood than neighbourhood-level economic indicators. The link between perception and reproductive timing provides support for the hypothesis that teenage motherhood may be an outcome of an evolutionary strategy linking reproduction to environmental conditions. This study also highlights the importance of using subjective perception of environmental conditions in models of human reproductive decision making, especially when ultimate, rather than proximate explanations are sought.
Dept. of Anthropology, Eliot College, University of Kent, Canterbury CT2 8AN
Email: s.e.johns@kent.ac.uk
Fertility decline driven by poverty in Addis Ababa (Ethiopia)
Eshetu Gurmu1 and Ruth Mace2
1Addis Ababa University, 2University College London
Historically, fertility decline is generally considered a response to development, and wealth and fertility have tended to be negatively correlated across countries. Here we document a rapid decline in fertility in Addis Ababa, Ethiopia, a country where recurrent famine and a stagnant economy have caused levels of poverty to increase. Multivariate analysis of the life history data of 2976 women of reproductive age revealed that low fertility in Addis Ababa is in response to poverty (i.e. scarcity of resources for parental investment). More than 50% of women currently in their 20s are predicted to fail to reproduce at all. Qualitative information collected through focus group discussions and in-depth interviews also support the statistical findings that poverty is at the root of this collapse in fertility.
1Demographic Training and Research Centre, Addis Ababa University, Addis Ababa, Ethiopia
Email: eshetu_gurmu@yahoo.com
2Dept of Anthropology, University College London, Gower St, London WC1E 6BT, UK
Email: r.mace@ucl.ac.uk