Human size has increased over the past century thanks to better health and nutrition, but this change has not occurred equally among men and women, a new study shows.
Men have grown taller and heavier at more than twice the rate of women, according to the study published this week in the journal, Biology Letters.
The researchers from Italy, the United States and the United Kingdom analyzed data provided in 2003 by the World Health Organization on the height and weight of more than 100,000 people across 69 countries. The study authors also used data from the Human Development Index (HDI), which measures national levels of human well-being.
The team found that each 0.2 increase in the HDI saw an increase in height of approximately 1.68 centimeters (0.66 inches) for women and 4.03 centimeters (1.59 inches) for men, as well as an average weight gain of 2.70 kilograms (5.95 pounds) for women and 6.48 kilograms (14.29 pounds) for men.
The trend was also confirmed by assessing data from the World Bank’s Gini Index, which measures national levels of income inequality, for 58 countries between 2000 and 2006.
Higher inequality was associated with decreases in height and weight. Each unit increase in Gini was associated with an average reduction in height of approximately 0.14 centimeters in women and 0.31 centimeters in men, and an average weight decrease of approximately 0.13 kilograms for women and 0.39 kilograms for men, according to the study.
While it could be thought that more developed countries could also simply have ethnic populations that are genetically taller, “we think that isn’t the case,” said study coauthor and environmental physiologist Lewis Halsey, a professor at the University of Roehampton who leads the Roehampton University Behaviour and Energetics Lab in London.
This is because the researchers found a similar trend when looking at a compilation of adult height data from just one country: the UK.
By analyzing the heights of 49,180 men and women between the ages of 23 and 26 from several UK studies published between 1905 and 1958, they found that women’s average height increased by 0.25 centimeters every five years, while that of men increased by 0.69 centimeters.
“This is one of the first studies to make a connection between the evolution of humankind as driven by sexual selection in combination with the effects of environment on ultimately our phenotype, so how we ultimately present ourselves, how we ultimately look characteristically,” Halsey told CNN on Wednesday.
Halsey believes the difference in the rate at which men and women are getting taller down to sexual selection. In the past, taller, heavier men would tend to be stronger, enabling them to outcompete other men, gaining more access to women and passing on their tall genes, he said.
However, even today, “women tend to prefer taller men,” he said, while, “in contrast, women’s height isn’t so important. So, to put it simply, men don’t tend to say, ‘Oh, I only like tall women.’”
“It is (a) nice cross-country study that basically confirms (an) already well known rule about the sex differences in ‘ecosensitivity,’” professor Bogusław Pawłowski, the head of the Department of Human Biology at the University of Wrocław in Poland, who was not involved in the study, told CNN Wednesday via email.
“As (the) ecological or economical situation improves and there is better access to resources, males gain more biological benefits than females. It is exactly opposite when the resources are scarce (males ‘suffer’ more than females).”
The “taller-male norm is something that occurs in Western countries (but also in many countries in Asia),” Pawłowski said. “It means that in these populations (a) man’s height is an important aspect of a man’s attractiveness on the human mate market.”
The researchers found that the variation in height among individuals of the same sex was lower in countries with better living conditions. And, as seen in the UK, within the population of an individual country, men’s height differences were smaller than women’s as living conditions improved over time.
This is because men, being bigger than women, “require more energy, they develop for longer” and, especially having more muscle, their tissues are “metabolically a bit more active,” Halsey said.
This means that for me to grow, “it takes longer and it’s more expensive, but that makes the male body more vulnerable to perturbations, to problems, to influences by the environment,” such as disease, he added.
So, when there is a stressful environment with more disease burden, men’s size is a lot more affected than women’s, he said.
Because men’s size is more sensitive to living conditions than women’s, the study authors concluded that men’s height and the differences in height between the sexes “may be especially useful biomarkers for tracking population changes in health.”
