I recently read that having large families may lower the IQ of the children. Is there evidence that such a claim is so?

Richard C. Galbraith, associate professor of Family Sciences, Brigham Young University. The “dumber-by-the-dozen” theory was popular among a group of scholars at the University of Michigan in 1975. In their mathematical model, known as the “confluence model,” a child’s intelligence was related to the average quality of that child’s family or home environment. The fewer the children, they reasoned, the more time parents would have for any one child. Children in small families do not have to compete for attention, while children in large families are often taught by their brothers and sisters, who are immature tutors. The authors of this theory therefore advised young couples to have only two children, and to space them far apart (at least five years).

A careful examination of the confluence model, however, discredits this entire line of thought. I will discuss only two points here. First, the mathematical formulas of the model do not produce the intellectual outcomes claimed by the authors. In our lab at BYU, as well as in the Michigan computer facility, scholars have not been able to replicate the confluence model; we find no support for the claims of its authors. In short, the confluence model was a mathematical mistake.

Second, a number of studies show that family size and child spacing have little or nothing to do with intelligence. In a study of 15,000 students at Brigham Young University, the number of brothers and sisters had no influence on college entrance scores. There was no evidence that coming from a large family was an intellectual handicap. In addition, there was no evidence that spacing children far apart in a family had any effect on their intelligence.

The confluence model no longer has popular support. Like many theories, it has been corrected over time. “The truth persists, but the theories of philosophers change and are overthrown. What men use today as a scaffolding for scientific purposes from which to reach out into the unknown for truth, may be torn down tomorrow, having served its purpose.” (Joseph F. Smith, Gospel Doctrine, Salt Lake City: Deseret Book Co., 1939, p. 39.)

(For a more complete and technical discussion, see Richard C. Galbraith, “Sibling Spacing and Intellectual Development: A Closer Look at the Confluence Models,” Developmental Psychology, March, 1982.)