“It’s not that we ‘could be’ the weaker sex — we are the weaker sex,” said Dr. Robert Tan, a geriatrics specialist in Houston who is on the advisory board of the Men’s Health Network. “Even when men and women have the same disease, we often find that men are more likely to die. Hip fractures stand out, for instance: women seem more likely to recover, while men are more likely to die afterward.”

Behavior plays a role in some of the extra deaths and illnesses among men: they tend to be more aggressive than women and to take more risks. Men smoke at higher rates than women, drink more alcohol and are less likely to wear seat belts or use sunscreen. Men also suffer more accidental deaths and serious injuries and are more likely to die of injuries and car accidents. They are three times as likely to be victims of murder, four times as likely to commit suicide and, as teenagers, 11 times as likely to drown.

Some experts think that depression contributes to these reckless and self-destructive behaviors, but that just as heart disease was initially defined by men’s experiences and therefore ignored or missed in women, depression may have been framed by women’s experiences and therefore may be missed and go untreated in men.

In any case, as a result, even though more baby boys are born, among people in their mid-30s, women outnumber men. Among people age 100, women outnumber men by 8 to one.

Among the questions research might explore, Dr. Legato said, are: “Why are there more miscarriages of boy fetuses? What is it about the sexing of the fetus that makes a male more vulnerable? What makes a boy less mature in terms of lung function after he’s born? And what is this propensity for risk-taking?”

One theory is that males are vulnerable because of their chromosomal makeup: where women have two X chromosomes, men have an X chromosome and a Y chromosome. “It is said that even before implantation in the wall of the uterus, the newly fertilized XX entity has a leg up,” Dr. Legato said, “because it can use that extra X to combat mutations in the chromosome that might be lethal or detrimental. And that might be a reason why females have a more sturdy constitution.”

Scientists and advocates who are concerned about men’s health are encouraging men themselves to take the first steps by accepting responsibility for their health status, seeking preventive care and making changes in habits, if necessary. New drugs for erectile dysfunction have helped bring men into doctors’ offices in recent years, experts say, but that is not enough.