In my early research several decades ago I considered the determinants of birth rates in the United States. I found, along with others, that higher income and more educated families had fewer children, that urban fertility was lower than rural fertility, that fertility tends to fall during recession (birth rates fell during this financial crisis), and that some other identified variables also had important effects on birth rates. But Catholic families at that time had considerably more children than Protestant or Jewish families with the same incomes, education, urban status, etc. The usual explanation for this was the obvious one: because of Church doctrines, many Catholic families in the United States were reluctant to use condoms or the other effective contraceptives that were then available.
A similar Catholic effect on fertility was also found in past international studies as well. Predominantly Catholic countries in Europe and elsewhere, such as Ireland, Spain, or Mexico, had larger families than did predominantly Protestant countries, like Sweden or Norway, even after adjusting for the effects on fertility of differences among countries in their average incomes, education, importance of cities, and other variables. Again, the explanation given for this result was that Catholic families were more reluctant to use contraception to reduce the number of children they had. Demographers even used the situation in Ireland to define a class of behavior called “Irish family patterns”, which meant that men and women married late-in their late twenties and early thirties- and that after marriage women gave birth at frequent intervals because couples made little effort to control their births once married.