The benefits of paid leave for children are real, majority of research says

(CNN) From the paid parental leave one-upmanship by Silicon Valley companies to Democratic presidential candidates calling for guaranteed paid leave to Republicans arguing mandated paid leave could drive small businesses out of business, the issue is getting more attention in the United States. Currently, the United States is the only industrialized country in the world that does not guarantee paid parental leave.

But what are the health benefits of paid parental leave on children, mothers and fathers? Are there real, tangible benefits, especially in the case of infants, of having a parent at home while their job is protected and their pay continues?

To answer that question, CNN reviewed more than 20 studies on the health impacts of parental leave on parent and child and talked to a handful of researchers. What CNN found is that most studies come to the same conclusion: Paid parental leave can have a significant positive effect on the health of children and mothers.

"Having paid leave makes economic and health sense," said Ariane Hegewisch, a study director for the Institute for Women's Policy Research, which released a report in 2014 summarizing the research on paid leave. "I think our report, in a way, supports a cost-benefit analysis of paid leave to say there are a lot of benefits."

Paid parental leave can reduce infant mortality by as much as 10%, according to a 2011 study of 141 countries with paid leave policies. It also increases the likelihood of infants getting well-baby care visits and vaccinations, with one study finding that children were 25.3% and 22.2% more likely to get their measles and polio vaccines, respectively, when their mother had access to paid maternity leave. Without paid leave, there was no increase in immunizations.

Paid parental leave can also increase the rate and duration of breast-feeding. A 2011 study in California found that women who had paid leave breast-fed twice as long as women who did not take leave. Babies who are breast-fed, according to the Centers for Disease Control and Prevention, are less likely to get a variety of infections and are also at lower risk for asthma, obesity and sudden infant death syndrome. There are benefits to mothers, too, as women who breast-feed are less likely to get breast cancer, ovarian cancer, type 2 diabetes and heart disease, according to the CDC.

Long-term benefits of paid leave?

There can be mental health effects, as well, of having job-protected, paid leave after the birth of a child. In one study, women who took longer than 12 weeks maternity leave reported fewer depressive symptoms, a reduction in severe depression and improvement in their overall mental health.

Another study, this one published this year, found that mental health benefits can extend over time. Women who were exposed to a more generous maternity leave policy were 18% less likely to suffer from depression 30 years later when they were 50 or older, said Mauricio Avendano, one of the study's co-authors and associate professor of social science, health and medicine at King's College London.

"The very significant message of this paper is that there is a potential for maternity leave benefit programs to have a real long-term effect on the mental health of women and that the effects of maternity leave benefits are not only short-lived ... they are likely to extend for many decades," said Avendano, who is also an adjunct associate professor for the Harvard School of Public Health.

Related: Work-life balance not just a women's issue

"This is really what economists call a human capital investment. You invest in this, you will end up picking up the benefits of this policy even years later."

For fathers, more involvement with child-rearing

While there is not as much research on the health impact of paid parental leave on fathers, there are effects on the family.

In a study looking at fathers in the United States, the United Kingdom, Denmark and Australia, dads who took paternity leave of 10 days or longer were more involved with their children and with child care activities than men who took no leave.

Ellen Bravo, executive director of Family Values @ Work, a coalition of groups in 21 states that tries to enact paid sick leave and paid family leave policies, pointed to Iceland, where each parent gets three months paid leave and then can split an additional three months leave. "They found that ... 70% of the men who take this time ... three year later, they are sharing child-rearing and we know all kinds of benefits for babies when they have this kind of involvement, not to mention the joy for the father," she said.

Not every study found a direct health benefit of extended paid parental leave. A study in Canadaexplored the impact after the country changed its maternity leave policies from six to 12 months in 2001. (Eligibility depends on a woman's employment history and hours worked, and pay is about 50% of her salary.) While researchers found that a lot of women took the leave and it expanded the amount of time women spent home with their children by about three months, they found little evidence the extra leave improved children's development or health.

"I can admit that I was a bit surprised by some of the results," said Kevin Milligan, professor of economics at the University of British Columbia Vancouver School of Economics, referring to finding no difference in terms of when children learn to walk, say their first words or feed themselves with parents who took extended leave and those who didn't.

But the results were less surprising, Milligan said, when they noted that most children with working parents in Canada still spend the first year of life with an aunt or uncle, grandmother or trusted friend or neighbor, rather than in a day care environment. "They're going to get the hugs and kisses. They're going to get the teaching to feed and teaching to say the first words pretty much as well as with Mom. So when you think about it that way, it's perhaps less surprising that we didn't find a big impact."

Researchers also said figuring out the right amount of leave is key. "It's not 'More leave is better,'" said Hegewisch of the Institute for Women's Policy Research. "There is a kind of balancing act between how much the mother and child may need and at what stage she may push out the father, and how much is too much."

Economic benefits, especially for women

When it comes to the economic benefit of paid leave, researchers have found it benefits women economically because they tend to go back to work and stay with the same employer, which means their wages grow at a faster rate afterward, said Hegewisch. There are also savings when it comes to turnover and training costs for businesses.

"There's not one of us who can say that we are as productive on Day 1 as we are on Day 366," said Bravo of Family Values @ Work. "Clearly, there's a getting-up-to-speed cost."

Opinion: Is Sheryl Sandberg right about Paul Ryan?

If more people were more aware of the health and economic benefits of paid parental leave, would there be more of a movement to make it a reality in the United States?

Bravo said she hopes so. Nearly one in four women who are employed go back to work within two weeks of delivery, she said.

"That's a national scandal. ... The good thing is unlike other kinds of national emergencies ... we know the solution for this. We know exactly what it looks like and we know what it takes to do it and we know that it works. All we need is the political will to get it done."

What do you think are the biggest benefits of paid leave? Share your thoughts with Kelly Wallace on Twitter @kellywallacetv or CNN Parents on Facebook.