We're used to seeing what's normal spread across a wide range of possibilities. In nature there is the large with the small, the tall with the short, the high with the low. The normal and the abnormal blend, often with no clear separation between them. Attributes that have disadvantages in one context have advantages in others. We can call this statistic monthly fertility .
Nature gives normal monthly fertility a wide range.
He calculated that the average normal chance of getting pregnant for a young couple living in the French countryside and embarking on a family was 20 percent per month. Arithmetic shows that the expected time such normal couples take to achieve a pregnancy was thus in the range of two to 18 months. But society has changed since the careful parish records of country England and rural France were put together in the 1800s. The result of these changes in society from 100 years ago is that because there are more older couples, we should halve the average monthly expectation of fertility from 20 percent to about 10 percent. This gives a normal range of monthly fertility in modern industrial societies of about 1.5 to 25 percent.