The Effect of Child Spacing on Previous Children’s IQ
| Effect of Gap Less Than or Equal to 1 Year | Effect of Gap Less Than or Equal to 18 months | |
|---|---|---|
| First child in families of three or more N = 74,996 | −0.26* (0.06) | −0.12* (0.02) |
| First and second child in families of four or more N = 32,772 | −0.18 (0.09) | −0.08* (0.03) |
| First three children in families of five or more N = 9,651 | −0.55* (0.17) | −0.19* (0.06) |
↵* indicates statistical significance at the 5 percent level. Standard errors (in parentheses) allow for correlation of errors within family. Each box represents a separate regression. The regressions include indicators for age, test year, mother’s age, mother’s age at first birth, mother’s education, father’s education, father’s IQ, birth order, and the log birth weight of the child studied. Also included is a control for family size and a quadratic function of the time gap between children born before the two children whose spacing is being investigated. The reported coefficients are those relating to the distance in years between the two children immediately after the children listed in column 1. For example, in the first box, the coefficient of −0.26 suggests that first child IQ is lower by 0.26 if the gap between the second and third child is less than or equal to one year (in families with at least three children).