To decrease the birth rate would require a big decrease in sexual activity. Here are the numbers: Researchers followed 881 couples through 7,017 menstrual cycles, with two stipulations for the volunteers: sex just once per month and no birth control of any sort. They found that on days outside the fertile window, conception occurred about 5 percent of the time, while during the fertile time around ovulation, the rate was 25 percent. Call the overall rate about 10 percent per unprotected intercourse. But protection with condoms or the pill or whatever brings the number way down—let’s say that rather than 10 percent, the rate with protection is 1 percent. That means to prevent one pregnancy an American would have to have sex 100 times less per year, just about a year’s worth of activity.