Generally, men are taller than women. To understand why it is so, scientists went back to Charles Darwin’s theory about competition for mates. In the book “The Descent of Man” Darwin mentioned that for ages men are the stronger gender, have broader shoulders, more developed muscles, and possess the courage needed for day-to-day struggle. This is considered as an evolutionary explanation for the height difference amongst genders. However, scientists did not stop with this theory and have continued digging deeper for more scientific explanations.
Evolutionary explanations for skeletal sex differences
Human sexual size dimorphism (skeletal sex difference) in height between males and females, is often advertised as a classic example of sexual selection. Louise Barrett, an evolutionary anthropologist at the University of Lethbridge, co-authored a review paper that explained the evolution of human height variation. In this paper, researchers stated that previous studies are mostly directed at understanding the difference in heights or development between humans and other primates. However, in most cases, the degree of shared evolutionary history has never been considered. Although sexual selection may answer why male and female chimpanzees differ in size, human dimorphism could be due to some other reason. For instance, it may be owing to their common ancestry or similarities with chimpanzees rather than the persistence of sexual selection. Barrett said that more research needs to be conducted for the advancement of our understanding of the sexual selection hypothesis. This would help in determining the evolutionary cause of height difference between sexes. She further added that dominance and competition could be the result of size differences rather than the cause.
In order to understand the genetic factors behind the different traits in men and women, researchers at the University of Helsinki, Finland, have reported the presence of genetic variations on the X chromosome, which is responsible for the height differences between genders. Male and female gametes (sex cells) contain either an X or a Y chromosome, where, females have two X chromosomes and males have one X and one Y chromosome. The researchers demonstrated that a genetic variant close to the ITM2A, a gene, involved in cartilage development, is frequent among people with relatively shorter height. The results suggest that the more the expression of the ITM2A, the shorter the person would be. This gene is present in the X copy of the chromosome, and women contain two copies of X chromosomes, meaning the gene is expressed with higher frequency in women.
Holly Dunsworth, a biological anthropologist at the University of Rhode Island, studied bone biology and hormones and correlated the two. According to her, the growth of the human bone depends on the hormone, estrogen. The production of estrogen is greater in females (ovaries) compared to males (testes). Therefore, a high level of estrogen in females results in a difference in height and mass and more pelvis expansion (due to estrogen and relaxin) compared to males. Dunsworth explained that ovaries play a crucial role in the development of the human skeleton. A greater amount of estrogen stimulates long bone growth and that’s why before puberty both genders grow at roughly the same rate. However, after the onset of puberty, in females, estrogen production is higher, which stimulates the growth plates in the bones, and thereby, causes the long bones to lengthen. This is also the reason for adolescent girls to be taller than boys. However, this spike in growth is short-lived. This is because high level of estrogen causes the growth plates to fuse and the onset of the menstruation cycle also slows down the production of estrogen in females. In contrast, adolescent men continue to grow for several more years until their estrogen peaks, as a result of which they become taller than females.
Link between Black Death and the height difference between genders
The Black Death was a bubonic plague pandemic that had claimed millions of Afro-Eurasian lives. Historians consider this as the most fatal pandemic recorded in human history. Researchers have claimed that after the pandemic, men gained around 9 centimeters in height, whereas, women got 5.5 centimeters shorter. The increase in height in men was perhaps because men were provided with better nutrition compared to females who did not receive adequate nutrition. Scientists have proclaimed that the development of height in adults is strongly dependent on nutritional intake and health during childhood.
Sharon DeWitte, an anthropologist at the University of South Carolina, contradicts the notion of a decrease in height in women owing to poor nutrition and ill health. She argued that the reduction in height in females following the Black Death might be due to an improvement in health or nutritional intake. An improved health condition is indicated by the early onset of menarche, as the age of menarche is related to the health condition. Thereby, it contradicts the hypothesis which states that sexual size dimorphism occurred due to competition.
The importance of knowing the right story
According to Dunsworth, common beliefs such as men are taller because of age-old competition and women’s hips are broader owing to childbirth are evolutionary stories. Although these stories are interesting, they bear consequences in our daily lives. For example, the sexual selection hypothesis sows a seed in our mind that men are born competitive and have to fight against their true nature just to keep up with the pre-existing notions. Barrett commented that it is of utmost importance to conduct extensive tests of evolutionary hypotheses, especially, while studying the evolution of humans.