Men appear to be evolving at double the rate of women, according to a major new analysis.
Researchers found that improved living conditions, such as better food, healthcare and fewer stressors, have resulted in men increasing in height and weight at faster rates, which carries on through generations.
Women, however, have not seen the same evolution, suggesting they are not as sensitive to environmental factors.
The study – which is titled ‘The sexy and formidable male body: men’s height and weight are conditfion-dependent, sexually selected traits – looked at the height and weight of more than 135,000 people across 62 countries from 2003 World Health Organization (WHO) data.
Experts compared the data to a measure of national levels of human well-being —the human development index (HDI) to see if factors in their environments played a role.
Individuals living in countries with low HDI scores are more likely to suffer from infectious diseases, chronic poor nutrition and larger overall disease burden than are individuals living in countries with high HDI scores.
Researchers found that for every 0.2 increase in HDI, men grew by 1.5 inches and gained 14 pounds of muscle.
The team said their findings among men aligned with the evolutionary theory where traits like height in men are seen as signals of health and strength, thus making them more attractive to potential mates.
‘Consequently, this is why men assess the formidability of potential opponents using physical cues, such as upper body strength, and women find taller, muscular men with a relatively large overall body mass (but not obese) particularly attractive,’ the researchers wrote.
Sexual size dimorphism (SSD), a noticeable difference in body size between males and females, is observed in many species like elephant seals.
The males weigh up to 10 times more than females.
The blanket octopus is an example of the roles being reversed – with females weighing up to 40,000 times more than males.
And most females, regardless of the species, tend to seek out the strongest, healthiest looking mate to with.
For the study, the researchers pulled 22-year-old data from the WHO from thousands of people who self-reported height and weight.
They also used the HDI for 2003 to measure the well-being of each country, which includes life expectancy, education levels and income.
For every 0.2 increase in HDI, women gained about six pounds, while men gained about 14 pounds.
An increase in height was also seen as women grew about 0.6 inches and men saw 1.5-inch increase.
‘Variability in weight was actually greater in nations with higher HDIs, and this might be explained by richer countries having access to more energy-dense processed foods enabling considerable weight gain by select individuals,’ the study reads.
Scientists also looked at UK specific data, finding men’s height increased faster than women’s overtime. Women’s height grew by 0.25 cm every 5 years, while men’s grew by 0.69 cm.
‘Our cross-national analyses suggest that as the social and ecological conditions of nations improve, including reductions in overall disease burden, people’s height and weight increase, but more than twice as much in men as in women resulting in greater SSD,’ the researchers wrote.
‘Based on our main finding of [sexual size dimorphism] being greater in more favorable environments, it is clear that the development and maintenance of gross morphology is more sensitive to living conditions in men than in women, at least in terms of height and weight.’
The scientists did not provide a HDI measurement that would see women evolve twice the rate of men, likely due to the gender not benefiting from improvements in living conditions in the same way as their counterparts.
‘SSD can result from the sexes occupying different feeding niches, larger females being more fecund or larger males having advantages related to female choice or male–male competition, researchers from the University of Genova wrote in the study.
‘The latter is common in terrestrial mammals, including humans. Men are on average taller and heavier than women across nations, which likely reflects an evolutionary history of physical male–male competition for status and resource control.’
They explained that a larger size, particularly coupled with greater muscle mass, comes with higher development and maintenance costs.
‘Full development of these traits is particularly dependent upon long-term physical and physiological health,’ the team shared.
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