Young men contemplating death seek to have more children, according to a study by researchers at LSE.
The paper Life After Death, published in the September edition of the Journal of Evolutionary Psychology, looks at the link between mortality and fertility by testing whether 'mortality priming' - questioning on death and dying - results in a marked increased desire for more children.
The study used 872 LSE students randomly divided into two groups. One group was asked 11 questions on death and dying before being asked several questions about having children. The second group was asked the questions on death and dying after the questions on children.
The number of children that male participants in the mortality primed group wanted was ten per cent higher than the number of children that male participants wanted in the control group, whilst there was virtually no effect for female participants.
Lead researcher Paul Mathews said:
'The shift in number of children that young men wanted after contemplating their own mortality was substantial. They went from wanting to have on average 2.29 children in the control group to 2.52 children after mortality priming - an increase of ten per cent.
'A similar increase in actual fertility in the UK population would result in a return to replacement level fertility (the level needed for population size to remain static, ignoring migration), which has not been seen in the UK since the 1970s. A rapid of upturn in the birth-rate such as this would be highly significant, resulting in considerable population growth and a slow down in population ageing.
This research was stimulated by evolutionary theory, since across species there is a well established trade off between the quantity and the quality of offspring:
'In an environment where there is lower life expectancy there will be an early start to reproduction and dilution of parental investment in a larger numbers of offspring - think of species such as frogs or rabbits.
'Conversely, where there is a higher life expectancy and stable mortality, there is later reproduction and smaller total number of offspring but with each individual offspring being provided with greater levels of parental investment - for example elephants.'
'The research shows humans adapt their response to different circumstances, as do other animals' said Mr Mathews.
But why was the reaction only found in men? The researchers argue this is driven by differences in reproductive physiology. A male only needs to conceive and then assume the mother will provide the necessary investment. For females such instant responsiveness is not viable, hence the difference in desired number of children.
Mr Mathews said: 'At every stage female reproductive physiology is more constrained than that of males - females can only conceive within a small window each month and during gestation they have to provide higher energy/calorific parental investment to the foetus for a sustained period of time. On the other hand, males only have to provide a single sperm.
'Human females are unlikely to simultaneously conceive and carry to term multiple gestations yet a male can father numerous offspring in a very short period of time through simultaneous gestation within numerous females.
'Finally, following birth and prior to weaning, human infants are highly dependant on their mother to breastfeed them.
'So it is clear that a females physiological capacity to respond to changing environmental conditions is relatively more constrained. What we have shown here is that these physiological differences seem to have substantial influence over the psychological reaction.'
Life After Death shows that what matters is not actual mortality but perceived mortality. For example in developing countries, urbanisation and increasing population densities might actually make people think they and their children are at greater risk, even if the reverse is true. This, in turn could have important consequences for the maintenance of high fertility.
Life after death: an investigation into how mortality perceptions influence fertility preferences using evidence from an internet-based experiment was written and conducted by Paul Mathews, PhD student and graduate teaching assistant in the Department of Social Policy at LSE and supervised by Dr Rebecca Sear, lecturer in population studies in the Department of Social Policy at LSE.
Click here to download a copy of the paper (PDF).
Paul Mathews on 020 7955 6552, 07817 969 987 or at email@example.com
Rebecca Sear on 020 7955 7348 or on firstname.lastname@example.org
Esther Avery, LSE Press Office, on 020 7955 7060 or at email@example.com
8 September 2008