The question: Does having children extend your lifespan? The U.S. birth rate recently hit its lowest point since 1920, arguably reflecting reports that many Americans are making the conscious decision to opt out of having children for personal and economic reasons. But these child-free Americans may be facing a trade-off, according to a new Danish study that examines the mortality rates of childless couples and parents — including adoptive ones — to see who ends up living longer. 

How it was tested: Researchers at Denmark's Aarhus University studied more than 21,000 couples trying to become pregnant via in vitro fertilization (assisted reproduction) between 1994 and 2005. Over this time period, 15,210 children were born and another 1,564 were adopted. The goal, says lead researcher Esben Agerbo, was to compare the death rate within this controlled population of willing parents to the death rate of childless couples. "Several previous studies have found strong associations between childlessness and psychiatric illness," Agerbo told NBC News. "I think that our study is superior, because it is only based on people who want to have children, whereas previous research included everybody."

The outcome: 96 women and 200 men died over the course of the study. Comparatively speaking, women without children were four times more likely to die a premature death than mothers, while men without kids were twice as likely to die than fathers. "Mindful that association is not causation," wrote researchers, "our results suggest that the mortality rates are higher in the childless." 

What experts say: Parents were found less likely to die from accidents, circulatory diseases, cancer, and other external factors. "Thus, I suggest a behavioral difference," said Agerbo. Perhaps non-parents are "more prone to buy a big motorcycle or a fast car than [a] family-friendly slow van." Or perhaps, conversely, parents are a little more willing to adopt healthier lifestyles in an effort to fulfill their child-rearing responsibilities. Agreed, says Alice Domar, a psychologist at Beth Israel Deaconess Medical Center in Boston. "When you have kids, it completely changes the way you live your life."

The lesson: This study lends credibility to previous research suggesting that having children — even if they do endlessly test your patience — can add years to your life.