Are you more likely to experience loneliness in early, middle or older adulthood? Here's what a new study says.

Loneliness has been a big topic in health over the past year, with the U.S. surgeon general, Dr. Vivek Murthy, comparing its negative health consequences to those of a daily smoking habit. Now, new research is breaking down when people are the most likely to feel lonely — and what other risk factors might also be at play.

The study, which was published in the journal Psychological Science, analyzed data from nine studies conducted in the United Kingdom, Germany, Sweden, the Netherlands, Australia, Israel and others. The researchers found that loneliness in adulthood follows a U-shaped pattern, where it's higher in younger and older adulthood but lowest in middle adulthood.

"Loneliness is a growing concern in the aging population," study author Eileen Graham, associate professor of medical social sciences at Northwestern University Feinberg School of Medicine, tells Yahoo Life. But Graham's work found that loneliness is more common in younger adults too. Here's what's behind this — and what other risk factors to be aware of.

Why might people be lonelier in early and older adulthood?

The study didn't specifically look at the reasons why this may happen, but there are some theories.

"In early adulthood, many individuals are experiencing shifts in life experiences," Hillary Ammon, a clinical psychologist at the Center for Anxiety & Women's Emotional Wellness, tells Yahoo Life. "Whether it be moving out of their childhood home for college or for a job, they are leaving a social network."

Young adults also tend to move to new cities away from families and even friends. "With less formal structure, it may be harder to form social attachments," Ammon says. "It requires more effort to make friends — looking into hobbies and signing up/showing up to events voluntarily." Ammon says she regularly hears from people in therapy who wonder how they can make friends as an adult. "It can be difficult to navigate that process," she says.

In older adulthood, it can be difficult to maintain social connections, especially if you move or "snowbird" someplace away from home, Thea Gallagher, a clinical assistant professor at NYU Langone Health and a co-host of the Mind in View podcast, tells Yahoo Life. "It's hard navigating the loss of friendships, and people you're friends with are passing away during this time."

There's also a "shift in life experiences" that can lead to feelings of loneliness, Ammon says. "Older adults will likely retire at one point, removing a structured environment for socializing," she says. "Physical mobility may be impacted as adults age. This may impact one's ability to leave the house or engage in hobbies that were previously enjoyed." Some older adults also say that they feel "forgotten" by their family members, "particularly their children, who may be busy with their own immediate families," she says.

Why are people less lonely in middle adulthood?

Graham points out that middle adulthood is "a uniquely complex and busy time" for many people. Gallagher agrees. "If you have kids, you have to be social because of them," she says. "You inevitably end up talking to people and making playdates, where you socialize."

People in middle adulthood are also likely to be more established in their area and have friends and neighbors where they work and live, even if they don't have kids, Gallagher adds.

That doesn't mean people can't be lonely in middle adulthood. "There may even be a period of loneliness for some who identify as new parents, as this can typically be an isolating period during which it may seem difficult to get out of the house," Ammon says. "Additionally, many new parents struggle to figure out what their identities and hobbies are after children."

But others in middle adulthood who are in the early parenthood stage may feel an increased sense of support from family and loved ones. "As children do get older, these adults may experience a greater sense of connection with others as a result of their children and their involvement in school and activities," Ammon says.

It's worth noting, however, that a recent study found that Americans in middle age are lonelier today than they were in the '90s and early '00s.

Risk factors for loneliness

The researchers found that age wasn't the only risk factor for being lonely. These factors also raised the risk of someone saying they were consistently lonely:

• Being a woman.

• Being physically isolated.

• Having less education.

• Having a lower income.

• Having functional limitations.

• Being divorced or widowed.

• Being a smoker.

• Having poor cognitive, physical or mental health.

"We tend to see higher levels of depression and anxiety in individuals that isolate," Ammon says. "We also know that people that tend to isolate experience more issues in terms of physical health, particularly related to weight and heart health."