"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.
The only mutations that matter to large-scale evolution are those that can be passed on to offspring. These occur in reproductive cells like eggs and sperm and are called germ line mutations. No change occurs in phenotype.
