Study shows that men who think about mortality aim to father more children
Only two events are inevitable in every human life – birth and death. But does our individual perception of one affect the other?
Research at LSE suggests it does – at least, if you are male – because young men contemplating death seek to have more children.
This was the finding of a study which looked at the 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 students randomly divided into two groups. One group was asked 11 questions on death and dying before being asked several questions about having children, including how many children they would like to have. 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.'
The findings of the study, which was supervised by Dr Rebecca Sear of the Social Policy Department, are also consistent with contemporary thinking in evolutionary psychology.
The research shows humans adapt their response to different circumstances, as do other animals' said Mr Mathews. '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 study's conclusions can also be supported by some non-experimental evidence. For example, a sustained rise in the number of births was recorded in Cambodia in the 10 years after the mass deaths of the Khmer Rouge. Similarly, though in a very different context, a rise in the birth rate was also seen in Oklahoma in the US after the 1995 bombings.
But why was the reaction to mortality-priming only found in men? The researchers argue this is driven by differences in reproductive physiology. A male only needs to conceive and then hope 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. Of course males do regularly provide substantial parental investment, but the minimum they need to invest is much lower.
'Human females are unlikely to simultaneously conceive and carry to term multiple children 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 may not be just actual mortality but how mortality is perceived. 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.
Now the researchers plan to widen their study, to test their hypothesis on more varied population samples. They will also investigate whether priming people to think not of their own or other adult deaths but of mortality in infants and children will produce different results.
Department of Social Policy