Barely-younger Siblings Distract From Reading

Each additional year of space between siblings helps the older’s reading scores by 1/6 of a standard deviation (about 3 IQ points). This is the equivalent of another $3000 of annual family income.

(Of course, this says nothing about what happens if you postpone additional children after hearing this fact - that may be different than what happens organically to make people temporally spread out their spawn).