Mecatornet:

In their 2011 State of Our Unions report (When Baby Makes Three: How Parenthood Makes Life Meaningful, and How Marriage Makes Parenthood Bearable) W. Bradford Wilcox and Elizabeth Marquardt found, like other researchers, that parenthood is typically associated with lower levels of marital happiness among contemporary couples. But that is not the whole picture by any means, as they explain in the following excerpt from the report, subtitled, "Family Size, Faith, and the Meaning of Parenthood".

Given the negative association between marital happiness and parenthood, one might expect that the least happy husbands and wives would be parents of large families. Not so.

In a striking finding, it turns out that the relationship between family size and marital happiness is not linear, but curvilinear (see Figure A1). In other words, according to the Survey of Marital Generosity, the happiest husbands and wives among today’s young couples are those with no children and those with four or more children.