From Sociologist Mark Regnerus:
To be sure, contraceptive usage prevents very many pregnancies—duh—but what it doesn’t prevent is all of them, given normal contraceptive failure rates (which vary) and the fact that many people don’t use them correctly (due to lots of reasons, ignorance being only one of them). But what I think typically gets left out of discussions about contraception—because it’s challenging to accurately discern it—is the effect on sexual decision-making of the wide social uptake of the Pill. One can argue whether it’s moral or not to use the Pill, or whether it’s immoral to deny access to it, but the Pill inarguably contributed directly to the single-largest drop in the “price of sex,” that is, how much relationship commitment is necessary (on average) before women agree to sex with men. (If you dislike this exchange mentality altogether and think it shouldn’t exist, well, you’re living in a dream-world.) This shift didn’t happen overnight; social change of such magnitude never does.
But it makes sense: take the risk of getting pregnant out of the equation (or in actuality, reduce the risk) and sex obviously will seem more advantageous and attractive to many. And it has. In other words, as the NYT focus on women in Lorain (Ohio) makes remarkably clear, in the era of the Pill people simply have sex in a nonmarital relationship more quickly than their grandmothers did, especially in their 20s. (I interviewed one college-educated woman last summer who tended to have sex on the first date if she didn’t think there was a future, but waited till the second or third date if she liked them and thought there was such a possibility—which so far as I can tell means “a relationship that lasts a while.” Marriage seems too much to hope for, although she would definitely like to be married someday.)
Add in the factors above—contraceptive failure rates and usage errors—and multiply by amount of sex that is going on and voila: you have more unexpected pregnancies than you anticipated, as a Nobel-winning economist documented over 15 years ago. It’s because the overall amount of sex occurring is greater, and the barriers to it much fewer, while contraceptive usage errors remain stable. Below is my own simple documentation of how wide uptake of the Pill can actually lead to comparable numbers of pregnancies and much greater nonmarital fertility than the pre-Pill era. (Note: this won’t be a completely-accurate formula, but rather my own guess-work).
Older pre-Pill model: 100 couples * 0.40 probability of an off-and-on premarital sexual relationship (due to fears about pregnancy risk, although probability that couples have ever had sex will be higher than 0.40) * 0.30 probability of pregnancy
risk in a year of frequent sex among a minority, infrequent sex among some, and little or no sex among plenty = 12 premarital pregnancies, and 10 marriages in a system wherein “shotgun marriages” are common = 2 nonmarital births (or 17% of births outside of wedlock).
Newer Pill-era model: 100 couples * 0.92 probability of a consistent sexual relationship (due to mating-market expectations of prompts sex and high confidence in contraception, which is unevenly practiced in reality) * 0.15 probability of pregnancy risk in a year of frequent sex among a mix of perfect, average, and poor contraceptive use habits across the 92 couples = 14 premarital pregnancies, only 4 of which become pre-term marriages = 10 nonmarital births (or 71% of births out of wedlock).