Wednesday 10 September 2003, 4pm, Quiet Room
Social class-specific life-history parameters determine women's reproductive strategies
Sara Grainger and RIM Dunbar, University of Liverpool
In the UK, family sizes tend to be larger, and women begin reproduction earlier, in the lower social classes. There has been considerable debate as to why this should be so, with two views predominating.
The social-economic perspective suggests that social and personal costs and benefits of reproduction (eg hindered education opportunities, increased welfare) are weighed up by women who then behave appropriately according to their individual circumstances.
The alternate view, inherent in educational and family-planning programmes, assumes that reproduction at young ages is usually accidental, with neither short nor long term benefits. Using a modelling approach parameterised with current UK life history data, we here show that a more likely explanation for earlier onset of reproduction in lower social classes may lie in higher class-specific mortality and infertility rates: in order to achieve the same final lifetime completed family size as women in higher social classes, women in lower social classes have to commence reproduction earlier. Such women cannot, therefore, afford to postpone reproduction in order to enter a career, unless that career facilitates upward mobility into a higher socio-economic class.
Email: rimd@liv.ac.uk
Email: Grainger@demogr.mpg.de
Teenage motherhood: an evolved reproductive strategy under conditions of environmental risk
Sarah E Johns, University of Bristol
A number of factors are correlated with teenage motherhood. However, the underlying causes of young pregnancy and birth remain elusive. Using data collected via postal questionnaire from young mothers in Gloucestershire, I have explored 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 personal future is uncertain.
Teenage mothers (12-19 years) were found to subscribe to a different life history trajectory - they had intercourse at a younger age, and expected to complete their families and die earlier - when compared to the older mothers (20-28 years). Additionally, women who perceived their pre-conception neighbourhood environments as being dangerous, or reported that their family life was disrupted during the transition to adolescence, had increased odds of being teenage mothers.
The results of this study suggest that women may embark upon different reproductive trajectories after menarche depending on the nature of their environment, and that perception of risk and danger is more important than economic measures in understanding reproductive decision-making. Additional work (presented elsewhere) indicates that the psychological mechanism of time perspective may link environment and behaviour in such decisions. This study is one of the first to test evolutionary ideas concerning age-at-first-birth in Western populations.
Email: sarahjohns@hotmail.com
Reproduction and longevity among the British peerage: the effect of frailty and health selection
Gabriele Doblhammer and Jim Oeppen, Max Planck Institute for Demographic Research and University of Cambridge
Why do humans age? Evolutionary theories suggest that organisms may either use their energy for maintenance and repair of their body functions or for producing offspring. For humans this would imply that life expectancy decreases with increasing number of children.
This paper shows that in a population with unrestricted fertility the relationship between number of births and life expectancy after the age of 50 is distorted by unobserved differences in health. Parents cannot be regarded as homogeneous - 'frail' women are probably selected into low parities and 'robust' women into high. This selection effect masks the true relationship and may explain the diversity of results in this field, where positive, negative and zero effects of childbirth on later survival have been found. If the effect of health is controlled in Hollingsworth's genealogy of the British peerage a significant trade-off between the number of children and life expectancy exists for females but not for males.
Max Planck Institute for Demographic Research
Konrad-Zuse-Str.1
18057 Rostock
Germany
Cambridge Group for the History of Population and Social Structure
Department of Geography
University of Cambridge
Downing Place
Cambridge CB2 3EN
Living long: theories of longevity in humans
John Lycett and Eckart Voland, University of Liverpool and University of Giessen
In this paper we review and assess theories of longevity in humans. Using data from the Krummhörn population in northwest Germany (18th and 19th centuries), we examine evidence for a number of recently reported associations between age at death and key life history events. We also consider a reported association between longevity and number of male versus female offspring, as well as seasonal effects of birth timing on age at death.
Email: j.e.lycett@liv.ac.uk