"This decline is broadly consistent with survey responses by mothers in 1973 who report that approximately 13 percent of lifetime births were unwanted [Statistical Abstract of the United States 1980, p. 65, table 99]. Note, however, that the decline in births is far less than the number of abortions, suggesting that the number of conceptions increased substantially—an example of insurance leading to moral hazard. The insurance that abortion provides against unwanted pregnancy induces more sexual conduct or diminished protections against pregnancy in a way that substantially increases the number of pregnancies. Another possible explanation for the gap between abortion rates and fertility rate changes is that illegal abortion was already suppressing the birth rate by 15–20 percent and legalization reduced it another 5–10 percent, but this would imply a higher figure for the number of illegal abortions than we think is likely, as discussed above."