1. Abstract

Evolutionary theories of parental investment suggest that parents face a trade-off between offspring quantity and quality, so that an increased number of children at the same wealth dictates negative consequences for individual children. Taking this prediction as a starting point, this study aims to 1) examine the consequences of high fertility on social and health aspects of child development, 2) examine the mechanisms, in terms of differential parenting behaviour, which lead to these consequences, and 3) investigate socioecological factors (namely kin support and wealth) which may relate to variation in the magnitude of effects across the British population. Finally, 4) the study will relate the findings of 3) to theories of fertility determination. When the effect of number of siblings is more negative, actual fertility is expected to relatively lower, and vice versa. All data will be sourced from the Avon Longitudinal Study of Parents and Children (ALSPAC), an ongoing extensive longitudinal database of children, and their parents, born in the Avon area of the UK.

2. Theoretical Background

Human evolutionary ecology is the study of the ecological and evolutionary basis of human behaviour, and the roles of behaviour in enabling humans to adapt to their diverse environments (Cronk, 1991; Voland, 1998; Winterhalder & Smith, 2000). A principle focus of human evolutionary ecology is patterns of human reproduction, and the life-history trade-offs this presents. Two main categories of life-history trade-off can be identified, and in an evolutionary sense, how one mother fares relative to another depends on how well these are handled over the lifecourse. The first trade-off is between current and future reproduction (Gadgil & Bossert, 1970; Williams, 1966), and is investigated empirically on the potential impacts of high fertility and early age at reproduction on a mother's survival and subsequent fertility (Perls, Alpert, & Fretts, 1997; Sear, Mace, & McGregor, 2003; Westendorp & Kirkwood, 1998). The second, and the focus of the proposed research, is between offspring number and offspring quality (Lack, 1947, , 1954; Lessells, 1991; Smith & Fretwell, 1974; Williams, 1966). Since both trade-offs involve the allocation of parental resources (time, wealth, energy) into alternative investments, and since an individual's resource budget is finite, these trade-offs are seen as inevitable.

The quantity-quality trade-off envisages children as competing for family resources, so that, controlling for parental resources, higher fertility will be related to negative consequences for individual children. Within developed countries evidence for this primarily comes from sociological studies of educational achievement. Across various measures of intellectual skills and educational success, individuals with the fewest siblings do the best according to studies that have used multiple datasets collected in both the United States (Alwin, 1991; Blau & Duncan, 1967; Circirelli, 1978; Downey, 1995; Featherman & Hauser, 1978; Kuo & Hauser, 1997; Mercy & Steelman, 1982; Parcel & Menaghan, 1994; L. C. Steelman, 1985; Zajonc & Markus, 1975), and Europe (Belmont & Marolla, 1973; Demographiques, 1973; Eysenck & Cookson, 1970; Majoribanks, 1974; Nisbet & Entwistle, 1967).

Comparatively little research has been yet carried out on health outcomes, at least within the developed world. There are however, a number of studies demonstrating a negative relationship between sibship size and child health and nutrition in developing countries (Ballard & Neumann, 1995; Hagen, Barret, & Price, in press; Hagen, Hames, Craig, Lauer, & Price, 2001; Nanda, 1996; Rao & Goplan, 1969; Wofle & Behrman, 1982). Given that other aspects of family structure, such as parental divorce, are known to hold important health consequences in developed countries (Mauldon, 1990; Quinlan, 2003) it seems likely that such effects may also occur in countries such as Britain.

Variation in the Quantity-Quality Trade-off

While the inverse relationship between number of siblings and educational outcomes has been heralded as one of the most consistent findings in the status attainment literature (Downey, 1995; L. Steelman, Powell, Werum, & Carter, 2002), there is a clear indication of variation in its magnitude across population subgroups. For example, Shavit and Pierce (1991) reported that, within the US, although sibship size had a negative impact on educational attainment for Ashkenazi Jews and Oriental Jews, it had no effect on Moslem Arabs. Similarly, Downey & Neubauer (1998) found the negative effects of sibship size were slightly weaker amongst Mormons than Protestants. Blake (1989) reports comparable differences between American Catholics and Protestants. Furthermore, the few studies on the consequences of high fertility on child education in less developed countries have proven inconclusive. Some studies have found a strong negative impact of family size on child outcomes (Birdsall, 1982; Knodel, Havanon, & Sittitrai, 1990), others have found it to be less important ((Mock & Leslie, 1986)) or even positive (Chernochovsky, 1985; Gomes, 1984). There are also a number of developing world studies that have failed to find a relationship between family size and child health and nutrition (Hesketh, Qu, & Tomkins, 2003; Tada, Keiwkarnka, Pancharunti, & Charoonsawasdi, 2002). Many authors have made suggestions in attempt to explain this variation, between and within studies, however, there has so far been little formal testing of the socioecological factors that may lie behind it.

One factor that may be important is the presence and relationship of extended kin to the children under study (Downey, 2001). Intuitively if sibship size reflects the dilution of familial resources amongst children, then measures of extended family structure may reflect the amount of resources available for dilution. Even at an equal parental wealth, spreading the costs of child raising with a stable partner or extended kin may lower the burden placed on the mother and so buffer the disadvantages of children from large families. Could it be then that the variation outlined above may be attributable to differences in the role of extended kin? Mormons for example have particularly strong profamily norms, as do Catholics (Blake, 1989). Developed countries are also characterised by a much more pronounced nuclearisation of the family relative to developing countries.

Human behavioural ecologists have a long standing interest in assistance of extended kin given to mothers in rearing children, as it hypothesised that this assistance may be responsible for the evolution of a number of unusual features of human life-history. These features include short interbirth intervals relative to other primates, and menopause (Hawkes, 2003). Thus, the contribution made from kin groups has been analysed across a wide range of historical and contemporary populations (Beise, 2004; Lahdenpera, Lummaa, Helle, Tremblay, & Russell, 2004; Marwole, 2001; Sear, Mace, & McGregor, 2000; Sear, Steele, McGregor, & Mace, 2002; Tymicki, 2004). What is clear from this body of research is that, while there is much variation in the role of particular kin across cultures, extended kin are often central figures in the development of children. Gibson & Mace (2005) for example demonstrate clear positive effects of assistance from grandmothers on child height and survival in rural Ethiopia. Thus these studies provide supporting evidence for the hypothesis that differences in the magnitude of sibship effects, both within and between societies, may be attributable to differences in the role of extended family.

Another factor that may lead to variation in the quantity-quality trade-off across population subgroups is socioeconomic class.Downey and Neubauer(1998) for example, have suggested that additional siblings adversely affects finances set aside for children in high income families, but has virtually no effect in low income families. They propose this is due to a difference in how number of siblings affects the distribution of base resources (i.e. those needed for survival) versus surplus resources (i.e. investments designed to enhance long-term opportunities). Only parents above a certain threshold of wealth are able to distribute surplus resources to children, so that parents below this threshold pay few costs to higher fertility as each child will only receive the minimum (base) resources in any case. A potential example of this effect could be drawn on schooling. All children of low income families will go to comprehensive school regardless of the number of siblings they have, however, in high-income families children may be able to go to private school depending on the number of siblings the family resources are to be diluted amongst. A similar effect could lead to differences in access to public vs. private health care.Downey and Neubauer's(1998) distinction between base and surplus resources is insightful and warrants serious thought and empirical investigation.

Theories of Modern Fertility

Understanding the socioecological factors that lead to variation in the costs associated with high fertility amongst British families also has direct relevance to our understanding of the determinants of modern fertility.

Among hunter-gatherer and other subsistence-based societies there is considerable evidence that fertility patterns reflect a set of physiological and behavioural responses that optimise levels of parental investment in such a way as to maximise Darwinian fitness (for reviews: Bentley, 1999; Kaplan, 1996). Evolved decision rules are hypothesised, allowing parents to strategically track the effects of parental investment on child outcomes and optimise their behaviour accordingly (Kaplan, 1996). However, given the phenomenon of low fertility, sometimes below replacement level, in post demographic transition societies some controversy rests on the role of these decision rules in determining modern fertility rates. In traditional societies wealth and fertility show a positive correlation (e.g. Low, 1990; e.g. Mace, 1996; Voland, 1990). Yet in those societies that have undergone demographic transition; that is, a period of history in which dramatic changes in fertility and mortality have occurred along with a rise in living standards, (Borgerhoff Mulder, 1998) wealth and fertility appear to show a negatively relationship (e.g. Lam, 1986; Perrusse, 1993). Furthermore, during the transition the rich have tended to reduce their fertility first. Why would people choose to limit their reproduction voluntarily when resources are plentiful? Some authors have suggested that the lack of a positive relationship between wealth and family size in modern societies is proof that humans no longer behave in a manner that optimises their reproductive success and that evolutionary approaches do not explain current family sizes (Vining, 1986).

Traditional methods of historical and economic demography, however, have failed to develop a robust theory of demographic transition. In response human behavioural ecologists have suggested a number of hypotheses to solve this dilemma (reviewed in(Borgerhoff Mulder, 1998). At the centre of these hypotheses lie changes in the quantity-quality trade-off, and they identify similar socioecological factors as those discussed above. For example, Turke (1989) and Draper (1989)both hypothesise that the reduced levels of fertility seen in modern societies may be a response to the absence of close kin networks. This is because strong extended family relationships decrease the costs of childrearing by providing free childcare and investment.Draper(1989) further suggests that even if the traditional role of extended kin has been replaced by friends or the government in modern societies, our evolved decision rules may still lead us to act as if extended kin are necessary for high fertility.

Other authors have suggested that suggested that increased wealth itself, and the increased opportunities this brings, may be lead parents to evaluating low fertility as the optimal strategy. As Kaplan et al.(1995) argues; a reduction in fertility would be strategically beneficial for wealthier individuals if the vale of investing resources in a child are a (positive) nonlinear function of the amount invested. This might happen, for example, because knowledge is cumulative: each additional unit of knowledge or skill acquired provides the platform for an even larger subsequent educational gain or access to much greater socioeconomic opportunities (as seems plausibly to be the case in our knowledge-based economies). However, these hypotheses remain somewhat speculative in the face of a lack of empirical studies into the relative costs of high fertility across subgroups in modern populations, and data on how these costs relate to actual family size.

3. Research Questions & Methodology

All data will be sourced from the Avon Longitudinal Study of Parents and Children (ALSPAC), an ongoing extensive longitudinal database of children, and their parents, born in the Avon area of the UK. The following research questions are identified:

A. What are the consequences of high fertility on child development in British families?

To date research into the effects of sibling number on child development has been heavily dominated by cross-sectional studies. Steelman et al.(2002), in a review of the sibling configuration literature to date, identify this as a area of major concern, not least because a recent large scale longitudinal analysis (Guo & VanWey, 1999) failed to replicate the negative impacts on child education that have been demonstrated in so many cross sectional analyses. ALSPAC then, provides the perfect resource to respond to these concerns, and estimate the effect of number of siblings on child outcomes with longitudinal data in Britain.

The nature of ALSPAC will allow for both educational and health aspects of child development to be considered. It is believed that analysing both within the same study will be extremely advantageous, as this will allow for a consideration of how different outcomes may be influenced by sibship size. Health measures will include growth (i.e. height), number of disease symptoms/infections (as a measure of general well-being) and the mother's opinion of the study child's general health. All these measures have been taken at regular intervals and are suitable for longitudinal analysis. Educational measures are taken less regularly and only towards the older ages in the ALSPAC study. It is proposed that the results of maths tests taken at 3 intervals by the study child will be investigated. However, at this stage of research planning it is unclear about the appropriateness or accessibility of these data. Alternatively then, child development scores may be used to assess cognitive development. The study will also consider the type of schooling the child receives (private/non-private). Parental resources (socioeconomic class, mother's general health) is an obvious potential confounding factor, and so will also be incorporated to the models.

B. Can differences in parenting behaviour be identified as family size increases?

In attempt to further our understanding of the mechanisms by which high fertility impacts on child development, an estimation of the effect of number of siblings on parenting behaviour will also be included in the study. Taking advantage of the rich nature of the ALSPAC dataset, proposed measures of parenting behaviour will include; parenting scores of mother and father figure; freq of visits to places of interest; entertainment, to see friends and family; number of books owned, and the amount of television watched. Once again these measures have been taken at regular intervals and are thus suitable for longitudinal analysis.

C. Can socioecological differences explain variation in the magnitude of "sibling competition" within the British population?

As discussed above, variation across population subgroups in the magnitude of sibship size effects has been suggested by several studies. Yet despite suggestions there has so far been little empirical testing of the socioecological factors that may lie behind this variation. The proposed study will investigate how measures of both kin and non-kin social support, and also parental wealth, relate to the magnitude of effects. Several proxies of kin support will be included: father presence/involvement with child, presence of grandparents; regularity maternal/paternal relatives seen; and grandparent/relative role in child care arrangements. Mother's membership of a close group of friends and opinion of neighbourhood will be included as proxies of non-kin social support. While standard measures of socioeconomic class and household income will be used to explore the influence of parental resources on the costs of high fertility.

D. Are higher costs of large family size on child development associated with lower actual fertility?

As a final research question, the study will consider how the costs of high fertility, in terms of child development outcomes, relate to the actual fertility decisions of the population. After identifying the relationship of social support and wealth to sibship size effects, a model of age-controlled fertility will be constructed entering these same socioecological factors. It is predicted that those socioecological conditions associated with larger costs of high fertility, in terms of child development (e.g. low social support/ high wealth), will be strong predictors of lower actual fertility. This prediction is based on the assumption that even parents in modern societies adaptively track the effects of parental investment on child development outcomes and respond accordingly.

4. Relevant Qualifications

1. Bachelor of Science (BSc) Hons, Biology University College London, 2003

2. Master of Science (MSc), Evolutionary Psychology University of Liverpool, 2004

3. Ph.D Candidate in Biological Anthropology University College London, expected 2008

4. During the early period of my PhD I will follow courses in these specialised statistics and the analysis of longitudinal data at the Centre for Applied Social Surveys (CASS), University of Southampton and at the UCL Graduate School. Multi-level models and event history models will be employed in the analysis of the ALSPAC data. Event-history analysis can be used to accommodate two common features of longitudinal event histories: censored observations and time varying variables. The inclusion of time-varying covariates in particular is crucial, since the key variables of interest in the study are the presence of kin during childhood which can clearly change over time.

5. References

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