BHPS-2009 Conference: the 2009 British Household Panel Survey Research Conference, 9-11 July 2009, Colchester, UK
June 1, 2009
Intorduction: In the modern gene centred view of evolution the notion of inclusive fitness is critical to explaining the reproductive behaviour of any organism. Natural selection favours characteristics which act to increase the frequency of genes in subsequent generations, and relatives by definition share genes. It is therefore often highly adaptive for organisms to aid and encourage reproduction in their relatives. Particular features of human reproductive physiology such as short birth intervals and the early cessation of female reproduction (menopause) have been argued to be evidence of cooperative breeding between relatives. This study will look to see if the proximity of relatives within females social networks increases their likelihood of having a first birth. Human behavioural ecologists have so far focussed on measuring inclusive fitness effects on the fertility of humans in natural fertility populations. As far as I am aware this study is the first to explicitly examine inclusive fitness effects in a low fertility population.
Data and Methods: The likelihood of a female having a first birth is examined using seven waves of the British Household Panel Study. The proximity of relatives within a social network is operationalised as the number of relatives who are within an individuals three closest non-household friends. Discrete-time event history analysis is undertaken to measure the likelihood of a first birth in an 18 month period after the interview, lagged by 6 months to control for pregnancy. Age, partnership status and household type are controlled for in the analysis.
Results: Provisional results show that each additional relative who is also considered a close friend significantly increases the probability of having a first birth. This suggests that women who have particularly close relationships with their kin have earlier first births than those with less close relationships with their relatives, thus providing support for the hypothesis that relatives will aid and encourage reproduction.