According to data released by the US Federal Bureau of Prisons last year, 93 percent of inmates in the US are men. The nation's incarceration rate peaked at 1,000 inmates per 100,000 adults during the three-year period between 2006 and 2008; the number was 310 in 1980.

When men are confined to jail or prison, or when a criminal record trails them following their release, they may become less attractive candidates for marriage.

A 2018 study conducted by the University of Nicosia in Cyprus, based on an analysis of 13,429 responses on Reddit, a popular social media website in the US, shows that the most frequent reasons that men indicated for being single included poor looks, low self-esteem and confidence, low effort, not interested in relationships, no available women and poor flirting skills.

"To be honest, sometimes I do get lonely, but it's never strong enough to make me think I wish I had someone," said Martin West, a 34-year-old self-employed small business owner in New York. "I do enjoy relationships, but being single has some perks that a relationship can't give you – so much disposable income, so much freedom in every aspect, no conflict or drama."

West said he has been single for more than eight years, and so far, he's still enjoying it. "And I've been through bad relationships — they are horrible."

He said his lifestyle is healthier when he is single, and he is also more focused on his own business than when in a relationship.

"But if I meet the right girl, then sure I'll lay my cards on the table and go all in," West added.

Just as West indicated, more studies now show that single people are becoming healthier, both mentally and physically.

A 2017 study published in the Journal of Women's Health found that unmarried women are living healthier than ever before. They had lower body mass indices, waist sizes and risks related to smoking and alcohol, compared with married individuals.

"A bad marriage is actually a health risk — people who are unhappily married have higher blood pressure and lower immune systems than happily married or single people," Coontz said.

She said that now that people are living alone longer and learning how to care for themselves instead of depending on a partner, they are practicing healthier lifestyles.

"For example, unmarried men used to dramatically be less healthy than married men, but recently they have come close to closing the gap," she said.

DePaulo said that in the US, there's also a lot of research showing that single people are more social than married people.

"They participate more in the events in their cities and towns, and do more to stay in touch with, and exchange help with, their parents, siblings, friends, colleagues and neighbors," she added.

The internet also allows single people to stay connected with others even when they are home alone, DePaulo said.