Early Childbearing and Behavioural Flexibility in the United Kingdom Abigail Chipman September ...

178 Pages·2017·1.6 MB·English

Early Childbearing and Behavioural
Flexibility in the United Kingdom

Abigail Chipman

Thesis submitted in partial fulfilment of the requirements for the award of the degree
of Doctor of Philosophy of the University of Portsmouth

September 2013
Abstract

Work in behavioural ecology has been and continues to be highly informative in the study of
human reproductive variation and behavioural flexibility. The traditional focus of this
approach has largely been on calibrations in reproductive behaviour in ultimate terms (i.e. a
specific behaviour is adaptive and maximizes fitness in different environments). Yet,
understanding the ultimate function of human behaviour as adaptive to local environments
allows for the deeper study of the proximate psychological, social and physiological
mechanisms that can shift both reproductive timing and corresponding reproductive ideals,
giving greater insight into the factors that influence early childbearing. Therefore, in the
thesis I aim to explore and confirm some of the mechanisms that impact on male and female
reproductive timing.
Firstly, I demonstrate that the impacts of social and environmental stressors such as the
local sex ratio result in different response patterns from women with different socioeconomic
backgrounds, the implication being that women with different life history trajectories have
different strategic responses to environmental conditions in line with the predictions of life
history theory. Secondly, I show that individuals’ subjective perceptions of their environment
are just as important, and potentially more important, indicators of their fertility intentions
than the often used objective indicators of environment quality such as deprivation. Thirdly, I
show that individuals take risks in strategic ways that can be explained by evolutionary
principles and that their future reproductive intentions are supported by pro-natal norms and
are not due to deficiencies in their knowledge of safe sexual practice. Fourthly, I consider the
evidence that kin networks help shape individuals’ psychology around life history strategies.
Finally, I explore the causal pathways by which acute stress shifts individuals’ life history
strategies and how this adjustment is moderated by an individual’s exposure to chronic
childhood stressors.

II
The findings resulting from this work merges with other research in the field of behavioural
ecology, moving towards an integrated understanding of human reproductive and
behavioural calibrations and exploring the ultimate and proximate questions of human
reproductive variation. These findings highlight the importance of understanding life history
trade-offs as central to reproductive scheduling. In addition it provides policy makers and
health workers with an alternative way of understanding early childbearing, one that sees
human behaviour within its adaptive evolutionary context.

III
Table of Contents

Abstract……………………………………………………………………………………………….II
List of Tables………………………………………………………………………………….......VIII
List of Figures……………………………………………………………………………………….X
Acknowledgements..............................................................................................................XI
Dissemination of Research.................................................................................................XII
Chapter 1. General Introduction ......................................................................................... 1
1.1 Overview ..................................................................................................................... 1
1.2 Biological explanations of reproductive variation and comparative evidence ............... 7
1.3 Human reproductive variation and early childbearing ................................................ 11
1.4 The social sciences and criticisms of an evolutionary perspective ............................. 14
1.5 Political perspectives and media representations of early reproduction ..................... 17
1.6 Reproductive variation in the United Kingdom ........................................................... 19
Chapter 2. The impact of sex ratio and economic status on local birth rates .............. 21
2.1 Overview ................................................................................................................... 21
2.2 Introduction ................................................................................................................ 22
2.3 Method and Materials ................................................................................................ 23
2.4 Results ...................................................................................................................... 25
2.5 Discussion ................................................................................................................. 27
Chapter 3. Fertility and parenting ideals in urban adolescents ..................................... 30
3.1 Overview ................................................................................................................... 30
3.2 Introduction ................................................................................................................ 31
3.3 Methods .................................................................................................................... 35
3.3.1 Participants ......................................................................................................... 35
3.3.2 Measures ............................................................................................................ 37
3.3.2.1 Personal networks ............................................................................................ 37
3.3.2.2 Environmental risk ............................................................................................ 38
3.3.2.3 Risk taking behaviour ....................................................................................... 39
3.3.2.4 Knowledge of safe sexual practice ................................................................... 40
3.3.3 Analytic strategy .................................................................................................. 41
3.4 Results ...................................................................................................................... 42

IV
3.4.1 Enviromental risk and age of first birth…………..…………………………………...46
3.4.2. Other main effects age of first birth ...………………………………………………..46
3.4.3 Interaction effects and age of first birth ……...……………………………….……...46
3.4.4 Environmental risk and desired number of offspring……………………………...…48
3.4.5 Other main effects and desired number of offspring……………………………...…50
3.4.6 Interaction effects and desired number of offspring………………………………....50
3.5 Discussion ................................................................................................................. 51
Chapter 4. Experimentally induced stress influences ideal female reproductive timing
........................................................................................................................................... 61
4.1 Overview ................................................................................................................... 61
4.2 Introduction ................................................................................................................ 62
4.2.1 Chronic stress and reproductive timing ............................................................... 65
4.2.2 Acute stress and reproductive timing ................................................................... 67
4.2.3 Research overview .............................................................................................. 70
4.3 Study One: Acute stress and life history preferences ................................................. 71
4.3.1 Methods .................................................................................................................. 72
4.3.1.1 Participants ...................................................................................................... 72
4.3.1.2 Design and procedure ...................................................................................... 72
4.3.1.3 Manipulation checks ......................................................................................... 74
4.3.1.4 Dependent measures ....................................................................................... 76
4.3.2 Results & discussion ............................................................................................... 77
4.4 Study Two: Acute stress, reproductive timing and more general effects .................... 80
4.4.1 Methods .................................................................................................................. 81
4.4.1.1 Participants ...................................................................................................... 81
4.4.1.2 Design and procedure ...................................................................................... 82
4. 4.1.3 Manipulation checks ........................................................................................ 82
4.4.1.4 Dependent Measures ....................................................................................... 84
4.4.2 Results & discussion ............................................................................................... 85
4.5 Follow up: Interaction between acute stress and chronic stress ................................. 89
4.5.1 Methods .................................................................................................................. 90
4.5.1.1 Participants ...................................................................................................... 90
4.5.1.2 Design and procedure ...................................................................................... 90
4.5.1.3 Results and discussion ........................................................................................ 91

V
4.6 General discussion .................................................................................................... 95
4.7 Limitations and future directions ............................................................................. 97
4.8 Conclusions ........................................................................................................... 99
Chapter 5. Observed sex ratios and sexual signalling behavioural change ............... 100
5.1 Overview ................................................................................................................. 100
5.2 Introduction .............................................................................................................. 101
5.3 Pilot One: Scanning male and female sexual signals ............................................... 106
5.3.1 Methods ................................................................................................................ 106
5.3.1.1 Operational Sex Ratio .................................................................................... 106
5.3.1.2 Contact with an unaffiliated female ................................................................. 106
5.3.1.3 Male behaviours ............................................................................................. 107
5.3.1.4. Female behaviours ........................................................................................ 110
5.3.2 Procedure ......................................................................................................... 113
5.3.3 Results & discussion ............................................................................................. 116
5.4 Pilot Two: Focusing on group male and female sexual signals ................................ 120
5.4.1 Methods ................................................................................................................ 120
5.4.1.1 Measures ....................................................................................................... 120
5.4.1.2 Procedure ...................................................................................................... 121
5.4.2 Results & Discussion ............................................................................................ 122
5.5 General Discussion ................................................................................................. 124
Chapter 6. General discussion ....................................................................................... 126
6.1 Overview ................................................................................................................. 126
6.2 Summary of main findings and implications ............................................................. 127
6.2.1 Sex ratios and behavioural variation.................................................................. 127
6.2.2 Subjective perceptions of the environment ........................................................ 130
6.2.3 Propensity towards risk taking and knowledge of safe sexual practice .............. 131
6.2.4 Kin networks ..................................................................................................... 133
6.2.5 Chronic and acute stress ................................................................................... 135
6.3 Future directions ...................................................................................................... 137
6.3.1 Investigating strategic behavioural variation ...................................................... 137
6.3.2 Policy implications ............................................................................................. 139
6.4 Conclusions ............................................................................................................. 141
References………………………………………………………………………………………...137
Appendix A.........................................................................................................................161

VI
Apprendix B........................................................................................................................166
Appendix C.........................................................................................................................178
Appendix D.........................................................................................................................187

VII
Declaration

Whilst registered as a candidate for the above degree, I have not been registered for
any other research award. The results and conclusions embodied in this thesis are
the work of the named candidate and have not been submitted for any other
academic award.

Word count: 36034

VIII
List of tables

Table 2.1 Parameter estimates with effect sizes for birth rate by age quintile. Data presented
from five generalized linear models. .................................................................................... 26

Table 3.1 Descriptive statistics for the main predictor and outcome variables ..................... 43

Table 3.2 Parameter estimates for ideal age of first birth from the generalized linear mixed
model .................................................................................................................................. 45

Table 3.3 Parameter estimates for desired number of offspring from the generalized linear
mixed model ....................................................................................................................... 49

Table 4.1 Mean (SD) subjective stress ratings and cardiovascular measures before (pre),
during and after (post) hand immersion in the warm water or cold water conditions ............ 75

Table 4.2 Descriptive statistics for variables not significantly different between the warm
water and cold pressor conditions. Data are means (SD) unless otherwise stated. ............. 79

Table 4.3 Mean (SD) subjective stress ratings and cardiovascular measures before (pre),
during and after (post) hand immersion in the warm water or cold water conditions ............ 83

Table 4.4 Descriptive statistics for variables not significantly different between the warm
water and cold-pressor conditions. Data are means (SD) unless otherwise stated. ............. 87

Table 4.5 2x2 analysis of variance for ideal age of marriage by acute stress condition, early
childhood stress and their interaction .................................................................................. 92

IX
Table 4.6 2x2 analysis of variance for ideal age of first birth by acute stress condition, early
childhood stress and their interaction. ................................................................................. 92

Table 5.1 Descriptive statistics (range, means and SDs) and inter-observer reliability scores
for observed male and female behaviours and the sex ratio for the first pilot study. .......... 115

Table 5.2 Significant correlations in pilot study one between the sex ratio and observed
behaviours. ....................................................................................................................... 117

Table 5.3 Descriptive statistics (range, means and SDs) for observed male and female
behaviours and the sex ratio for the second pilot study. .................................................... 123

X