Why males are more likely to die from conception to old age
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Sexual inequality begins in the womb, but not in the way you might think. In a in South Australia between 1981 and 2011, we found boys are more likely to be born preterm and the risk is greater for boys the earlier the birth.
Mothers expecting boys than mothers of girls to suffer pre-eclampsia (a serious disorder of pregnancy characterised by high blood pressure, fluid retention and protein in the urine), gestational high blood pressure or gestational diabetes late in pregnancy.
Many more boys are conceived than girls. Despite this the sex ratio at birth is only slightly in boys’ favour. 106 boys are born, a statistic that . But males are more likely to die before females at all ages from conception to old age, which we think explains why Australia is around 51% female despite fewer girls being born.
Miscarriages and stillbirths are . After birth, or suffer major illness. data show boys make up 75% of SIDS deaths, 54% of cancer diagnoses, 60% of infant deaths and are more likely to be disabled (often associated with preterm birth).
As males and females age, are prevalent in Australia. Greater numbers of men suffer heart disease (59%), endocrine disorders including type 2 diabetes (57%) and cancer (56%).
. These include blood and metabolic disorders (59%), neurological disorders including dementia (58%) and musculoskeletal conditions including arthritis (56%). There is also a female predominance in many auto-immune diseases.
Why are men more likely to die earlier?
We don’t know for sure why there are differences in disease prevalence, severity, age of onset and even symptoms between the sexes, but genetic differences between males and females could contribute to differences in the uterus.
Males have XY sex chromosomes and females have XX sex chromosomes. We found 142 genes are expressed differently between normal male and female placentas delivered at term. About a third of the genes are on the sex chromosomes, but two-thirds are on the autosomes (non-sex chromosomes) and are associated with hormones.
The greatest sex differences are in the brain, specifically in the anterior cingulate cortex, which controls things such as heart rate and blood pressure as well as some emotion and decision-making (1,818 genes), followed by the heart (375 genes), kidney (224 genes), colon (218 genes) and thyroid (163 genes). In other organs, sex differences were mostly confined to genes on the sex chromosomes and those involved in hormone production.
Since defects in how the placenta develops and functions are associated with pregnancy complications, it is likely the placenta is a key contributor to the different outcomes we see between pregnancies carrying boys versus girls. These probably hark back to our evolution.
Evolution and the battle of the sexes
In the animal kingdom, males are somewhat dispensable, with the dominant male the most likely to breed with multiple females each season. Thus, in many species, it is only the biggest, strongest and fittest males who reproduce.
Bigger babies are more likely to survive birth and infancy and grow up to reproduce. So maintaining fetal and post-natal growth makes the male more likely to pass on his genes.
Females, conversely, will almost always reproduce and pass on their genes – assuming they survive to adulthood. So the growth strategies of the male and female fetus focus on passing on their genes to the next generation.
sex differences in how the fetus responds to maternal asthma. Asthma attacks in pregnancy, which are akin to an inflammatory storm, cause the female fetus to taper her growth in response. In so doing, the female fetus is more likely to survive.
However, a maternal asthma exacerbation does not affect the growth of the male fetus. He keeps growing at the same rate but places himself at risk of preterm birth and stillbirth should another asthma attack occur.
The thesis links the growth and development of the fetus to the health of the infant, child and adult. How well we grow in utero strongly influences our propensity for adult onset diseases. The fetus is said to be programmed in utero for health or disease across the life course.
So how well you grow in the uterus is influenced by your genetics but also by environmental factors. Together these shape your health for life and sex matters.
, Senior Research Fellow,
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