Breast Cancer and Early First Pregnancy?

Reading David Freedman’s book “Statistical Models: Theory and Practice” today, I was very struck by this passage:

Example 1. In cross-national comparisons, there is a striking correlation between the number of telephone lines per capita in a country and the death rate from breast cancer in that country. This is not because talking on the telephone causes cancer. Richer countries have more phones and higher cancer rates. The probable explanation for the excess cancer risk is that women in richer countries have fewer children. Pregnancy — especially early first pregnancy — is protective.1

Is Freedman correct about the protective benefits of pregnancy? This would be remarkable if true. And, in the absence of evidence to the contrary, I am likely to believe Freedman’s claims.

  1. David Freedman : Statistical Models: Theory and Practice : Chapter I