Counties in the United States with some of the steepest increases in home values are also showing the steepest declines in fertility rates, with people who can afford expensive homes increasingly either not being able to afford to have children or choose not to until after they're 30-years-old, according to a recent study.

If one county's home values increased 10 percent higher than another county's did, the fertility rate in the first county also fell by 1.5 percentage points, the study, compiled by Zillow.

"A lot of the economic rationale for having kids has fallen away," Eric Kaufmann, a professor at Birkbeck College at the University of London, told Axios.

The average age for a first-time home buyer has gone up from 32.5 years of age in 2013-2015 to 35.2 years old in 2017, and women are also waiting longer to have children.

Meanwhile, the age group of people most likely to consider having a baby, but who do not already own a home, are people between 25 and 29 years old, notes Zillow, while the average age of mothers at birth has gone up from 27.7 years old in 2010 to 28.7 years of age in 2018.

According to a 2015 report from the U.S. Department of Agriculture, housing expenses are both the largest cost for raising children, as well as the fastest growing.

Zillow reports that this could affect families needing more space or who want to buy a home before they have children.

The news comes as fertility rates have dropped in the United States in recent years, Axio notes,

particularly during the 2008 financial crisis.

Home values have gone back up, but birth rates continue to fall, according to Global Aging Institute President Richard Jackson.

"Millennials are more risk averse than earlier generations at the same age," said Jackson "People 50 or even 25 years ago didn’t wait to be ‘financially well established’ before starting a family. Now it’s considered irresponsible not to.”

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