Actually, the first happiness studies that found that having children massively decreases happiness were using involuntarily infertile couples, not voluntarily childfree folks, as their comparison group; the authors were very surprised that involuntarily infertile childless couples were happier than their child-having peers!

A few of these early studies: Glenn, N.D., & McLanahan, S. (1982) Children and marital happiness: A further specification of the relationship. Journal of Marriage and the Family, 44, 63-72 (great quote: negative effect of offspring on both marital and global happiness of parents "is not absolutely conclusive, of course, but it is perhaps about as nearly conclusive as social scientific evidence on any topic ever is."

Anderson, S.A., Russell, C.S., & Schumm, W.R. (1983). Perceived marital quality and family life-cycle categories: A further analysis. Journal of Marriage and the Family, 45, 127-139.

Bernard, J. (1982). The Future of Marriage. New Haven: Yale University Press.

Campbell, A., Converse, P.E., & Rodgers, W.L. (1976). The quality of American life: Perceptions, evaluations, and satisfactions. New York: Russell Sage.

Campbell, A. (1981). The sense of well-being in America. New York: McGraw-Hill.

Elderly childfree are happy too: Rempel, J. (1985). Childless elderly: What are they missing? Journal of Marriage and the Family, 47, 343-348.