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Human Nature Review 2002 Volume 2: 302-305 ( 22 July )
URL of this document http://human-nature.com/nibbs/02/grant.html
Maternal Personality, Evolution and the Sex Ratio: Do Mothers Control the Sex of the Infant?
by Valerie J. Grant
Reviewed by Mark Sergeant, Graduate School for Social Research, The Nottingham Trent University, Burton Street, Nottingham, NG1 4BU, United Kingdom.
In animals that reproduce via sex there is an equal birth (secondary sex ratio) of one hundred males to one hundred females (100:100) which serves to maintain the balance between males and females in the population; in humans the secondary sex ratio is one hundred and five males to one hundred females (105:100). It also appears that the human sex ratio is prone to fluctuations; one well documented example is the increased number of male births recorded during and after the first and second world wars, among the populations of those countries directly involved in the conflict (MacMahon and Pugh, 1954). Given that the number of X and Y bearing spermatozoa from a male are equal, and therefore should result in an equal sex ratio, how can these effects in the human populations be explained?
Grant’s book seeks to explore the influence that maternal personality can have on the human sex ratio, with a particular focus on the Maternal Dominance Hypothesis. The basic idea behind this is that dominance levels in females have a direct effect on the sex of any offspring they reproduce. More dominant females are alleged to produce more male offspring and less dominant females produce more female offspring. The function of this difference is similar to the hypothesis put forward by Trivers and Willard (1973) that females will produce the offspring that has the greatest chance of carrying their genes into the next generation. For animals in good condition (relating to food and resource availability) this will be males, who can potentially have more offspring by fertilising several females, but for animals in poor condition this will be female offspring, who have less numerous offspring but more guarantee of reproduction. Since the production of the original paper more focus has been given to dominance being the crucial factor, though good condition and dominance are connected, but not identical, as dominance often equates with good conditions.
Grant draws extensively on animal based research (due to the more basic research ethics involved and the focus on physiology to explain behaviour) to support her claims concerning humans. The link between female dominance and sex of offspring is more fully documented in other mammalian species such as deer (Clutton-Brock et al, 1984) and rhesus monkeys (Meikle et al, 1984). In humans meta-analysis performed on six studies run between 1969 and 1991 has demonstrated that dominant women have a significantly higher number of male offspring (Grant, 1996).
The concept of dominance is well explored in chapter two of the book, with Grant adopting Fiske’s (1971) definition that “Dominance can be identified as acting overtly so as to change the views or actions of another”. Dominance is identified as a non-bipolar and independent personality characteristic that is distinct from being aggressive or domineering, two terms that are often incorrectly equated with dominance. The level of dominance exhibited by a female is positively correlated with their levels of testosterone; higher levels indicating a more dominant individual, additionally these levels are not static but are instead in a feedback relationship with external stimuli (stress is detailed below), so they fluctuate over time. Testosterone levels have been shown to increase when an individual increases in dominance, both in terms of both physical and mental activities (Mazur and Lamb, 1980; Mazur et al, 1992).
Particular attention is given to the effects of environmental stress, which can raise testosterone production in women through stimulation of the adrenal cortex, where a large percentage of female testosterone originates. Elevated testosterone causes increased dominance as females “toughen up” under times of stress. Therefore “Tough times mean more males” as the stress associated with these “tough times” elevates testosterone levels, leading to increased dominance, which in turn leads more women to produce male offspring. This has the potential to explain why a greater number of males were born during the two world wars; the stress of a prolonged conflict combined with an increase in female dominance originating from some women taking over traditional male occupations.
Grant believes that these differences in dominance and associated testosterone levels increase or decrease the likelihood of an X or Y bearing spermatozoa, for dominant and non-dominant females respectively, to fertilise the ovum. Grant theorises that there may be a testosterone threshold that must be surpassed to ensure male offspring, with continuous variation in maternal testosterone levels explaining why the majority of women are just as likely to have female as male children as they are just under or over the threshold respectively. The precise biological mechanism that may be involved in this selection process is currently unknown, but Grant draws on work by Saling (1991), involving reproduction in mice, to suggest it may be linked to a filtration or sorting process that the cumulus matrix and zona pellucida perform. Spermatozoa may be unable to penetrate these layers of the ovum unless they meet some form of ‘selection criteria’. This process is unproved in humans but could act as a viable mechanism for the maternal control of the primary and secondary sex ratios.
One thing that is evident throughout the book is Grant’s emphasis on scientific research methods in the production of her theory, based on the fact that “in this area of science there have been so many false claims” (p 6). A good example of this is Grant’s consideration of how dominance should be defined and measured (chapter 2), which involves considering the process of researching personality characteristics and highlighting its inherent advantages and limitations. This emphasis continues throughout the book when considering the validity of other researchers work and its relationship to her theory. For example Grant considers the work of Shettles (1970) to be problematic, and points to France and Graham (1984) as being more “scrupulous in designing their own study“ (p 65) therefore producing more valid results. It is Grant’s attention to the research methods used by previous investigators that supports her conclusions drawn from the available data.
In chapter 10 Grant considers the debate surrounding the deliberate pre-selection of an offspring’s sex by prospective parents, resulting from a strong cultural preference for a child of a certain sex, usually male. Grant quotes the example of South Asia, notably India, where there is a preference for sons because of the need for a daughter to be supported in marriage through a dowry, effectively meaning that “Economically a female child is considered a drain on the family purse” (Ramanamma and Bambawale, 1980). Among some families this has led to an interest in predetermining the sex of offspring; Boom, Verma, and Beri (1994) highlight an incidence of this information being used for sex selective abortions in the Punjab region. Such a decision would not be made lightly however, with individual parental conscience being crucial, and most women would still see abortion as an occasion of “fear, if not dread” (p. 189). Such selective control of an offspring’s sex could result in an unbalanced sex ratio (British Medical Association, 1996) that would have long term implications for the society in question, particularly how females would be perceived if there was a preference for male offspring.
This debate is relevant to the current situation in China where the idea of single child families, a preference for male offspring, and the use of sex selective prenatal screening has led to a severe bias in the national sex ratio (Junhong, 2001). It is estimated that these factors have altered the secondary sex ratio to around one hundred and seventeen males to every one hundred females (117:100). As a result of this it is estimated that by 2020 between 29 and 33 million men may be unable to find wives, possibly leading to a variety of social problems, such as an increased incidence of prostitution in cities and the ‘trafficking’ of women in some rural areas (Hudson and Den Boer, 2002). Grant concludes that one way to cope with this issue is not to focus on the pre-selection of a particular sex, but instead to redefine the value that is associated with characteristics of the two sexes, a view that is shared by other researchers (Junhong, 2001).
Grant has produced a well thought and coherent hypothesis on the effects that maternal personality could have on the human sex ratios. The literary format of the text provides what can be complex data in an interesting and informative manner that will engage with both an academic and general audience. The core concepts of the maternal dominance hypothesis are well explored throughout the book and are supported by valid research findings, though some areas of the book are not currently as substantiated or convincing as others. Grant’s considerations of the ethics and implications of attempts to alter human sex ratio by artificial means also make very engaging and thought-provoking reading. In conclusion I believe the text would be of interest to a variety of audiences, from evolutionary psychologists to reproductive physiologists, because of the breadth of empirical work studied and the consideration given to the implications of the theory and peripheral issues involving the human sex ratio. In short a well written book and highly recommended.
British Medical Association (1996) Medical ethics today: Its practice and philosophy, Latimer Trend & Co, Plymouth
Booth, B. E., Verma, M., & Beri, R. S. (1994) Fetal sex determination in infants in Punjab, India: correlations and implications, British Medical Journal, 309, 1259-61.
Clutton-Brock, T. H., Albon, S. D., & Guinness, F. E. (1984) Maternal dominance, breeding success and birth sex ratios in red deer, Nature, 308, 358-60.
Fiske, D. W. (1971) Measuring the concepts of human personality, Chicago, Aldine.
France, J. T., Graham, F. M., Gosling, L., & Hair, P. I. (1984) A prospective study of the preselection of the sex of the offspring by timing intercourse relative to ovulation, Fertility and Sterility, 41, 894-900.
Grant, V. J. (1996) Sex determination and the maternal dominance hypothesis, Human Reproduction, 11, 2371-5.
Hudson, V. M., & Den Boer, A. (2002) A Surplus of Men, a Deficit of Peace: Security and Sex Ratios in Asia's Largest States, International Security, volume 26, Issue 4.
Junhong, C. (2001) Prenatal sex determination and sex-selective abortion in Central China, Population and Development Review, volume 27, no 2.
MacMahon, B., & Pugh, T. F. (1954) Sex ratio of white births in the United States during the Second World War, American Journal of Human Genetics, 6, 284-92.
Mazur, A., & Lamb, T. A. (1980) Testosterone status and mood in human males, Hormones and Behaviour, 14, 236-46.
Mazur, A., Booth, A., & Dabbs, J. M. (1992) Testosterone and chess competition, Social Psychology Quarterly, 55, 70-7.
Meikle, D. B., Tilford, B. L., & Vessey, S. H. (1984) Dominance rank, secondary sex ratio, and reproduction of offspring in polygynous primates, American Naturalist, 124, 173-88.
Ramanamma, A., & Bambawale, U. (1980) The mania for sons: an analysis of social values in South Asia, Social Science and Medicine, 14B, 107-10.
Saling, P. M. (1991) How the egg regulates sperm function during gamete interaction: facts and fantasies, Biology of Reproduction, 44, 246-51.
Shettles, L. B. (1970) Factors influencing sex ratios, International Journal of Gynaecology and Obstetrics, 8, 643-7.
Trivers, R. L. & Willard, D. E. (1973) Natural selection of parental ability to vary the sex ratio of offspring, Science, 179, 90-1.
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© Mark Sergeant.
Sergeant, M. (2002). Review of Maternal Personality, Evolution and the Sex Ratio: Do Mothers Control the Sex of the Infant? by Valerie J. Grant. Human Nature Review. 2: 302-305.