What does research say about the relationship between birth order, sibling spacing, and intelligence?

"Birth order and sibling spacing are unrelated to a child's intelligence."

Explanation:

The theory, put forward by Robert Zajonc, a psychologist at the University of Michigan, holds, in brief, that the greater the number of children in a family and the shorter the time between their births, the lower will be the intelligence of the children, particularly those born later. A mother's genetics determines how clever her children are, according to researchers, and the father makes no difference. Women are more likely to transmit intelligence genes to their children because they are carried on the X chromosome and women have two of these, while men only have one.