When we see a tall, muscular guy, able to lift and carry heavy weights, run fast up the stairs or run the Yukon Quest, we think of men as the stronger sex. We confuse stronger muscles with general physical robustness.
But the research is showing that in many ways men are actually more fragile than women.
The weakness of males is obvious even before birth.
If a woman has a miscarriage the fetus is more likely to be male, reports Ruth Nass in "Sex Differences in Learning Abilities and Disabilities."
Obstetrical complications such as toxemia of pregnancy are more common with male fetuses (1.7 to 1), as are placenta abruptio (2 to 1), spontaneous abortion (1.4 to 1) and birth trauma (1.8 to 1).
Boys are also born with more physical maladies than girls. They are more apt to suffer virtually every neurodevelopmental and psychiatric disorder of childhood.
In an interview about her book "The Sexual Paradox," Susan Pinker puts it this way:
"Males are more vulnerable to maternal stresses and pollution in utero -- female preemies are almost twice as likely to survive as male preemies.
"Boys are twice as likely to have attention problems, four times as likely to have language or reading disabilities, and 10 times more likely to have Asperger syndrome.
"Males are more susceptible to almost all chronic diseases, including heart disease, cancer, liver disease and AIDS. They have much shorter life spans.
"They are more aggressive and take more risks, which is one reason why there are more male prisoners (the ratio of male to female prisoners is 10 to 1) and male suicide.
"Victims of work and school violence are 93 percent male.
Males catch postsurgical infections more than women do, and more die from these infections."
While the gender gap is declining, women still live longer than men, on average about five years.
But why is this the case? In his new book "Male, Female, The Evolution of Human Sex Differences," David Geary suggests an interesting explanation for some of these differences. Due to male competition and fighting with other males for females in traditional societies, males needed to grow large and muscular.
Males may also have needed to grow large and muscular because they specialized in activities that require substantial strength, like hunting and warfare.
Such greater growth requires a longer time of maturation. Males grow more slowly than girls but continue to grow for a longer time. Males reach puberty later, for example, and males reach their adult heights at a later age.
"Boys are born 'premature' relative to girls," according to Geary. "They have higher activity levels and higher basic metabolic rates than girls, resulting in higher caloric requirements for normal development."
Boys are more vulnerable while they are maturing.
In well-nourished populations of boys with caring parents attentive to their health and mindful of the risks if their boys take dangerous chances, the greater vulnerability of males is not obvious, Geary argues.
But put both boys and girls in poor conditions and the greater vulnerability of males will surface. One study examined the health of 22,873 infants to 6-year-olds in New York City, children who came from families that were not well off. Thorough physical examinations showed that 64 percent of the girls were in good health compared to 58 percent of the boys.
Furthermore, eight times as many of the boys had multiple disorders.
Another study followed very-low-birth-weight boys and girls for two years. During the first year, 41 percent of the boys died but just 19 percent of the girls. And twice as many of the surviving boys were likely to have medical and developmental problems.
This is not to say that girls don't have higher rates of other problems, like anxiety, depression, suicide attempts and eating disorders.
But the next time you see a tough and muscular guy, keep in mind that, yes, they may be tough and muscular, but you can also look at males as the more fragile sex, especially vulnerable to all kinds of developmental disorders and earlier death.
Judith Kleinfeld is a professor of psychology and co-director of Northern Studies at the University of Alaska Fairbanks.