One of the most common methodological flaws in abortion research involves the use of poor comparison groups. A good comparison group will match the abortion group as closely as possible, controlling for a number of other factors - age, income, education, and so on. One of the most important factors to control for is the wantedness of the pregnancy. When studying the psychological effects of aborting an unwanted pregnancy, the only adequate comparison groups will be composed of women who carried an unwanted pregnancy to term.

Skewing the results against abortion is as easy as choosing an inadequate comparison group. Examples of this include:

  • Women who have never been pregnant.
  • Women who carried a wanted pregnancy to term.
An example:

A 2006 study by Fergusson, Horwood, and Ridder concluded that women who aborted were at an increased risk of mental health problems. The "pregnant - no abortion" comparison group was not controlled for pregnancy wantedness or intendedness - there is no way to know how many of these women were actively trying to get pregnant and viewed the pregnancy positively. Furthermore, this study was conducted in New Zealand, where abortion access is extremely limited:

Firstly, women must approach their doctor and are then referred to specialist consultants. Two certifying consultants must then agree: 1) that the pregnancy would seriously harm the life, physical or mental health of the woman or baby; or 2) that the pregnancy is the result of incest; or 3) that the woman is severely mentally handicapped. An abortion will also be considered on the basis of age, or when the pregnancy is the result of rape. (p. 17)

The "pregnant - abortion" group and the "pregnant - no abortion" group are composed of two very different sets of people. The first includes only women requiring abortion for medical/psychological reasons (some of whom may have been deeply emotionally invested in their pregnancies), rape/incest victims, very young girls, and mentally handicapped women. The latter includes women who wanted their pregnancies and women who were forced to carry their unwanted pregnancies to term due to legal restrictions on abortion. It's impossible to know how many of the participants fall into each of these categories within the two groups.

This obviously casts a great deal of doubt on the validity of the results. The failure to control for pregnancy wantedness and extremely traumatic events - such as rape or the termination of a wanted pregnancy due to severe medical complications - makes it seem absurd to draw conclusions about the abortion procedure as a primary cause of mental illness in the participants. In a section titled "the role of contextual factors," the authors weakly identify (and mostly skirt around) this serious problem with their research:

It is clear that the decision to seek (or not seek) an abortion following pregnancy is likely to follow a complex process relating to: a) the extent to which the pregnancy is seen as wanted; b) the extent of family and partner support for seeking or not seeking an abortion; c) the woman's experiences in seeking and obtaining an abortion. It is possible, therefore, that the apparent associations between abortion and mental health found in this study may not reflect the traumatic effects of abortion per se but rather other factors which are associated with the process of seeking and obtaining an abortion. For example, it could be proposed that our results reflect the effects of unwanted pregnancy on mental health rather than the effects of abortion per se on mental health. The data available in this study was not sufficient to explore these options.

I'll give you a moment to place your bets on how anti-abortion groups portrayed the results.

Done? Okay.

LifeNews reported that several medical journals rejected the study, but in nearly 500 words failed to find the space to even mention the methodological problems that might've led to the decision.

"We went to four journals, which is very unusual for us — we normally get accepted the first time," he said.

Fergusson told the Herald he knew of the "circus" that could result from the study but he said it would be "scientifically irresponsible" to not publish the results even though they are controversial.

Hear that? It's the world's smallest violin, and it's playing just for you.

In an article titled "How Abortion Harms Women's Health," the Family Research Council included this blurb:

A "pro-choice" research team in New Zealand, analyzing data from a 25-year period and controlling for multiple factors both pre- and post-abortion, found conclusively that abortion in young women is associated with increased risks of major depression, anxiety disorder, suicidal behaviors, and substance dependence. This is the most comprehensive, long-term study ever conducted on the issue.

Right. Well, except for the fact that the authors admitted that there were other plausible explanations for their findings, and that the editors of a number of peer-reviewed journals felt that the research was too weak to warrant publishing it. I really do hope that this isn't "the most comprehensive" study "ever conducted" on the topic.

Right to Life of Northeast Ohio, Voice for Life, and The National Right to Life Committee (NRLC) also commented on the study. None of them mentioned the flawed comparison groups. The NRLC claimed that the study "brought to light the professional bias" against results that "challenge the pro-abortion political agenda."

It seems that Fergusson had no trouble shoving the weaknesses in his work under the rug - and unfortunately, there are even more subtle ways of manipulating comparison groups to dramatically alter results. One popular method involves excluding participants with a history of multiple abortions from the delivery group but not the abortion group. The result? Women who have had no abortion are compared with women who've had any number of abortions. It might seem unlikely that this would significantly impact results, but (for reasons mentioned below) it can actually make a big difference.

In fact, a 2005 study by Schmiege and Russo found that attempting to mitigate this effect by excluding participants who'd had abortions subsequent to their first pregnancy from both the delivery group and the abortion group had a profound effect on the results. Women who delivered were significantly more likely to be depressed (determined by exceeding the cut-off score for Center for the Epidemiologic Studies Depression Scale, or CES-D) than women who had an abortion (APA Task Force on Mental Health and Abortion, p. 38-41). When women who'd had subsequent abortions were included in both groups, there was no significant difference in depression rates between the two.

Compare this to studies that do differentially exclude women with subsequent abortions from the delivery group (but not the abortion group!) and you'll find that almost all of them show higher levels of mental illness in the abortion group.

In other words, something as simple as removing women with subsequent abortions from the delivery group without doing the same for the abortion group can dramatically bias the direction and significance of the results. This is a very nuanced way to bend science, and if you don't know to look for it, it's easy to miss.

The likely explanation for this effect is actually quite simple: lower levels of education and higher rates of poverty correlate with repeated unwanted pregnancies (APA, p. 33). Additionally, pregnancy coercion and contraceptive sabotage correlate with a higher risk of both intimate partner violence and unwanted pregnancy (Miller, Decker, McCauley, Tancredi, Levenson, Waldman, Schoenwald, & Silverman, 2010). As the APA Task Force notes in its overview, "positive associations observed between multiple abortions and poorer mental health may be linked to co-occurring risks to both multiple unwanted pregnancies and mental health problems."

To summarize: comparison groups should always be as closely matched to abortion groups as possible. If researchers can't provide a very compelling reason for manipulating their comparison group(s) in ways that subvert their similarities with the abortion group, doing so presents a serious threat to the validity of their research.

If this post has piqued your interest in the methodological flaws that taint most abortion research, I can't encourage you enough to read the Report of the APA Task Force on Mental Health and Abortion in full.