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The propagandist hopes you will not think too carefully, the persuader asks you to consider. Persuasion changes people’s thinking; propaganda reduces their ability to think. It would be easy, given the urgency of the pro-life cause, to propagandize rather than persuade, but it also ultimately would be ineffective, because minds changed by propaganda can easily be changed by better propaganda.

How then, can we persuade our fellow citizens of the merits of protecting the unborn? Cognitive research shows how our attitudes about an issue can be strongly predicted by how likely we think certain possible results of an action (like changing the legal status of abortion) will be and also by how bad or good we think those results will be.

For example, those who think that making abortion illegal will cause women to seek “back alley” abortions and who view this outcome as a great evil will likely be pro-choice. Those who think that making abortion illegal will save the lives of innocent babies and consider this result a great good will likely be pro-life. The pro-life movement can persuade others most effectively not by arguing philosophically for the humanity of the unborn (though those arguments must be made), but by battling misinformation about the nature and likelihood of specific results of abortion.

In 1995 the sociologist Kristina Petkova asked students at the University of Massachusetts, hat would happen if abortion became illegal? Those who identified themselves as pro-choice thought that making abortion illegal would result in what they believed to be a “bad” result: in women seeking “back alley” abortions and in the infringement on fundamental human rights of privacy and freedom of choice. It would also result in teenagers paying a high price for being careless, which they considered a “moderately bad” result.

Pro-choice students also thought it moderately likely that making abortion illegal would result in the birth of babies conceived in incest or rape (which they considered bad), the birth of unwanted children (moderately bad), exposure of families to the risks of poverty and welfare (moderately bad), and the birth of “retarded and genetically unfit babies” (only slightly bad).

What did pro-choicers consider to be less likely or even unlikely to happen if abortion were made illegal? They thought it slightly likely that making abortion illegal would save the lives of unborn babies (which they considered to be a slightly good result), make more babies available for adoption (moderately good), and hold people more responsible for the consequences of their actions (also moderately good).

The pro-choice students’ response to the question about saving the lives of unborn babies deserves special mention. They believed outlawing abortion would be only “slightly likely” to save lives and that it would be only a “slightly good” result if it did. This differs sharply and strikingly from the opinion of pro-lifers, who view saving babies as a likely and a good result of making abortion illegal.

The main insight to be gained from this study is that when pro-choice students”and pro-choicers in general”think about making abortion illegal, they base their decision on subjective judgments that bad things will happen if it is made illegal and that the good things pro-lifers predict will happen won’t. This suggests that pro-life attempts to persuade them might fruitfully focus on countering their judgments about the effects of the action or about the value they place on it, or both.

We might, for example, attempt to puncture the inflated misperceptions of the likelihood of “back alley” abortions and pregnancies resulting from rape and incest. We could argue that restriction of the freedom to abort”like the restraining of other “me first” freedoms”will result not only in good for the child, but also for the mother.

Research into people’s attitudes about abortion also points to a basic asymmetry between pro-life and pro-choice positions: Pro-lifers and pro-choicers differ in the degree to which they have considered their own and opposing arguments. Pro-choicers tend to be less aware of challenges to the pro-choice position than pro-lifers are aware of challenges to the pro-life position. This means that pro-choicers are more susceptible to simple challenges.

In a 2001 study by psychologists Ronan Bernas and Nancy Stein, pro-choice and pro-life students read four of eight possible cases in which a woman was seeking an abortion. Each case challenged either the pro-life or the pro-choice position. One-third read the four cases supporting their own position, one-third read the four cases challenging their own position, and the final third read two of each.

The cases challenging the pro-life position described a woman seeking an abortion because her physical health and life were in danger if the pregnancy continued, she was psychologically unstable and suicidal during pregnancy, she was pregnant by rape, or she was pregnant by incest. The cases challenging the pro-choice position described a woman seeking an abortion who had had multiple abortions in the past and was using abortion as birth control, whose life was in jeopardy if she underwent abortion, who wanted an abortion because the unborn was of an undesired sex, or who wanted an abortion because her husband threatened to leave her if she carried the pregnancy to term.

The results were enlightening. Pro-choice students changed stances more often than pro-life students did, largely because pro-life students had already considered challenges to their own position, whereas pro-choice students tended not to have done so. The attitudes of pro-choice students were more susceptible to challenge because they had not critically evaluated them.

These results suggest that persuasive pro-life strategies ought to include multiple simple challenges to the pro-choice position that are unlikely to have been considered. To begin with, pro-life conversationalists and media campaigns can use the challenges presented in this study, revised slightly: What about cases where the abortion is coerced by a boyfriend or mother or father? Where the unborn child is aborted because she is a girl? What about those times when abortion is being used repeatedly as birth control or when it jeopardizes the life of the woman?

But one’s position on abortion is rarely only a matter of knowledge. It also involves personal, and typically emotional, interests. People want to maintain their current attitudes because they feel change to be a threat, especially to their sense of self or to their self-oriented interests. People who feel threatened and defensive resist persuasion. Liberal attitudes toward sex outside marriage, for example, are correlated with approval of abortion on demand. People who have assisted others in getting an abortion are also more strongly pro-choice.

People may be better able to bear a threat to their sense of self from one area if their sense of self is first shored up”if, for example, they have an opportunity to affirm personal characteristics important to them. In a 2007 study by the psychologist Geoffrey Cohen, pro-choice people who were asked to assume the role of a Democratic legislator conceded more to a pro-life negotiator when they had just written about a personal characteristic that was important to them, like their sense of humor or their creativity. Paradoxically, they were more willing to make pro-life concessions when negotiating with a pro-lifer in conditions where their pro-choice identity was made salient.

The insight to be gained here is that in attempts at persuasion it is important to take into account underlying defensive motivations. One cannot ignore defensive sentiments and rely solely on cognitive challenges or logic.

Finally, those who seek to persuade others need to work through relationships. A person’s attitudes are influenced by the number of people influencing him, by the strength of their influence over him, and the closeness of their relationships with him. Over time, this results in the geographical or spatial organization of attitudes and beliefs. People in these powerful echo chambers keep hearing the same arguments over and over, while people with minority attitudes or beliefs remain silent or exit.

This insight suggests an important, missionary-like strategy in pro-life work: Develop messages specifically for different pro-choice groups. Examples of this type of strategy are Feminists for Life, Democrats for Life, and University Faculty for Life. Building on their common identity, values, and relational forces, each of these “indigenous” groups is most able to influence others in their group in a pro-life direction.

And because attitudes often stem from what people think their group’s stance is (for example, “I’m a Democrat, I must be pro-choice”), simply knowing that the group is diverse tells them that a faithful member of the group can dissent (“I can be a pro-life Democrat because these people are pro-life Democrats”). This permits people to maintain core identities that are too important to them to change, while changing some particular beliefs.

One final thought: Getting people to think about an issue more deeply by “staying the course” is key to changing minds. Civil-rights advocates, for example, persisted in calling for equal rights despite enduring contempt, hatred, and even violence. They grounded their movement in biblical injunctions of equal dignity for all human beings, values that the people they were trying to persuade held, though they did not apply them to civil rights. Many people who held them formed their opinions privately at first but later reached a “tipping point,” at which they were willing to express their support for civil rights openly as this position became socially acceptable.

Consistency despite the cost, and an appeal to common values, caused Americans to consider the issue more deeply. A courageous witness proves to be the greatest persuasion.

Nicholas DiFonzo is professor of psychology at Rochester Institute of Technology and author of The Watercooler Effect (Penguin). A version of this essay was delivered at the 2011 University Faculty for Life conference.