Legalized abortion is a bigger factor than broader access to birth control in the declining U.S. birthrate and steadily shrinking family sizes, according to new research by a Middlebury College professor.

“The results suggest that policies governing access to the pill had little, if any, effect on the average probabilities of marrying and giving birth at a young age,” Middlebury College professor Caitlin Knowles Myers wrote in a December report for the Journal of Political Economy that received little notice in the mainstream media.

“In contrast, policy environments in which abortion was legal and readily accessible by young women are estimated to have caused a 34 percent reduction in first births, a 19 percent reduction in first marriages, and a 63 percent reduction in ‘shotgun marriages’ prior to age 19.”

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There were 3,999,386 infants born in the United States in 2010, 3 percent fewer than in 2009, according to the National Institutes of Health. The decline was seen in nearly all races and Hispanic-origin groups.

Myers traced the court decisions and legislative changes beginning in the 1960s and 1970s that removed legal obstacles to birth control and abortion for couples and single women. She compared that information to data gathered by the U.S. Census Bureau.

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Anti-abortion activists said the study points to the profound and underappreciated impact that widespread abortion has had on the U.S. population.

“When you’re killing a million babies a year, obviously you’re going to have a major effect on birth rates,” said Jim Sedlak, executive director of the American Life League.

“Maybe it’s ideology driving all of these things, and not policy.”

Other researchers said they believe birth control availability has had a greater impact than the Middlebury College study suggests.

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“There’s some reason to be skeptical of it,” said Michael New, who teaches economics at Ave Maria University near Naples, Florida.

New, who also is an associate scholar at the Charlotte Lozier Institute, said a fairly large body of research contradicts Myers’ conclusion. He said the conventional wisdom is not always right but should be considered when evaluating an outlier study.

New noted that the Centers for Disease Control estimates that 90 percent of American women use birth control at some point during their lives. He said that far exceeds the number of women who get abortions. Abortion rights activists peg that number at one in three. He said he believes that overstates the true number, but even that high estimate falls well below birth control rates.

“Contraception is much, much more common among women than abortion is,” New said.

But Myers argues in her paper that any reductions in unwanted pregnancies among teenagers resulting from increased use of birth control were offset by large increases in sexual activity. She notes that the failure rate of the birth control pill is about 9 percent during the first year of typical use.

In fact, the study states, greater use of birth control may actually have increased the birth rate.

“There is a large gap between rates of sexual activity and use of family planning services, suggesting that it is improbable that large fractions of women were substituting from condoms to the pill,” the paper states.

Myers contends that, while the pill “fueled the sexual revolution,” it was abortion that allowed women to delay marriage and children.

New said there are flaws in drawing conclusions based on changes in the law. Laws, he said, can be circumvented. He said changes in birth control and abortion also probably are related. He said more liberal states — the ones most likely to expand access to birth control and abortion before the Supreme Court’s Roe v. Wade decision in 1973 — also are more likely to have large numbers of feminists less inclined toward marriage and children.

“Maybe it’s ideology driving all of these things, and not policy,” he said.

Patrick Fagan, director of the Marriage and Religion Research Initiative at Catholic University in Washington, said the Middlebury study raises a good question. But he said determining whether abortion or increased use of birth control has had the greater impact is beside the point.

“To me, it’s a false dichotomy making it an either-or,” he told LifeZette. “I think it’s really a ‘both-and’ … Once you take sex out of marriage, it doesn’t matter which way. You get a massive change in family structure.”

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New said he suspects the Middlebury study will gain a lot more attention in the coming months as abortion supporters use it to argue that making the procedure legal has benefited society. Indeed, Myers cited past research crediting the pill with increasing educational attainment, improving marital stability, improving long-run outcomes for children, reducing crime, and increasing female participation in the workforce.

New said many of those assertions do not hold up under scrutiny. He said neither abortion nor the pill reversed the trend toward out-of-wedlock births or reduced child abuse.

Fagan said more than half of women’s first-born children enter the world without a married mother and father. He said that trend “has all sorts of consequences” that are not easily addressed.

“It’s much easier to break something than it is to repair it … Once you break the culture, you’ve altered it,” he said.

PoliZette senior writer Brendan Kirby can be reached at [email protected]. Follow him on Twitter.