The gender gap in life expectancy is widening — in women's favor. Here are 5 reasons why.

Women have long been projected to outlive men — not just in the U.S. but around the world, both in more developed and underdeveloped countries. Now that gender gap appears to be growing stateside, with women expected to live nearly six years longer than men, according to new research.

The study looked at the contributions of COVID-19 and other underlying causes of death to the life expectancy gender gap from 2010 to 2021. It found that the difference in how long men and women were expected to live increased by 0.23 years from 2010 to 2019 and 0.70 years from 2019 to 2021. The gap widened to almost six years in 2021 — the largest difference since 1996 — with a life expectancy of 79.1 years for women and 73.2 years for men, according to data from the Centers for Disease Control and Prevention (CDC).

Meanwhile, the life expectancy at birth in the U.S. continued to drop overall — from 78.8 years in 2019, to 77 years in 2020, to 76.1 years in 2021.

Historically, a big reason for women’s longer life spans has been differences in smoking behavior, leading women to have lower cardiovascular and lung cancer death rates than men, according to the study.

Dr. Michael Fredericson tells Yahoo Life that there’s evidence that, in general, women tend to adhere to healthier lifestyles than men. Fredericson, who was not involved in the study, is co-director of the Stanford Longevity Center and director of Stanford University’s lifestyle medicine program.

“I think women will probably always be ahead, but I think we can do a better job catching up now that we've come out of COVID,” Fredericson says.

COVID-19 had a major impact on men’s life expectancy, according to the study; here are five things researchers identified as factors that are widening the gap in how long men and women can expect to live in the U.S.

1. COVID-19

From 2019 to 2021, COVID-19 led to a 0.33 year difference in life expectancy between men and women — making it the biggest contributor to the widening gender gap. Overall, men experienced higher COVID-19 mortality rates than women; in 2021, there were 131.31 deaths per 100,000 men versus 81.66 deaths per 100,00 women.

The study says there were likely multiple reasons for this, including more comorbidities in men and socioeconomic factors like incarceration and homelessness. More men may have also participated in the labor force outside the home — putting them at greater risk of infection during the pandemic; according to the U.S. Census Bureau, women made up a majority of at-home workers during COVID.

2. Unintentional injuries

Unintentional injuries were the second-biggest contributor to the widening life expectancy gap, with a 0.27 year difference between men and women; prior to COVID-19, unintentional injuries were the leading contributor.

The drug overdose epidemic was a major culprit, according to the study, with accidental poisonings (mostly drug overdoses) accounting for most of the unintentional injuries. Men are more likely than women to engage in illicit drug use (e.g., illegal drugs or the misuse of prescription drugs), and men are more likely than women to die from or need an emergency department visit for illicit drug use.

3. Heart disease

There’s been a “persistent gap” in heart disease between men and women, according to the study, and prior to COVID, heart disease was one of the leading contributors to worsening life expectancy for men.

Heart disease is the leading cause of death in the U.S. and refers to several types of conditions, including the most common, coronary artery disease (CAD), which can lead to a heart attack.

Men often develop heart disease earlier than women. Women have higher levels of the reproductive hormones estrogen and progesterone, which may offer some protection in young adulthood by boosting blood vessel health.

Fredericson says lifestyle factors may also be at play: “Smoking, high blood pressure, being overweight — these things all affect your cardiovascular system, your metabolic system, having higher cholesterol, and all those things are going to put you higher at risk for cardiovascular disease or really all diseases that could affect your life expectancy.”

4. Suicide and homicide

“The increase in overdose deaths, homicide, and suicide underscore twin crises of deaths from despair and firearm violence,” the study's authors said.

Though women are more likely to attempt suicide, more men die by suicide — and guns are a major reason why, with firearms accounting for 54.64% of all suicide deaths in 2021. That same year, men died by suicide 3.9 times more than women, with white males comprising nearly 70% of suicides.

“In general, men are more prone to using a firearm than women would be,” Fredericson says. “They're probably more likely to have a gun and to have access to a gun.”

5. Diabetes

Men are more likely to have undiagnosed diabetes and because they tend to have more belly fat, men are also more likely to get type 2 diabetes at a lower weight than women.

Non-insulin-dependent diabetes (aka. type 2 diabetes) usually appears after age 40 and is the most common type, accounting for up to 95% of diabetes patients.

“Type 2 diabetes is very preventable if you modify your lifestyle, and I think women do a better job of that than men,” Fredericson says.

In fact, in addition to lowering the risk of diabetes, Fredericson says there are several commonsense habits anyone can make to improve life expectancy overall, including:

• Not smoking

• Keeping your body mass index between 18.5 and 25.

• Getting at least 30 minutes a day of moderate to vigorous exercise

• Limiting alcohol intake

• Eating a healthy diet that is low in saturated fats and processed foods and is heavy on the fruits and vegetables

“If men can do a better job with incorporating healthy lifestyle factors into their life, I think we can do a better job sort of catching up to women in terms of life expectancy,” Fredericson says.