A new study in the Journal of the American Medical Association says that the ideal spacing for babies is at least 18 months, but fewer than five years. Too close together and your risks of low birth weight go up, at the rate of 1.9% per month. Too far apart, though, and you start to run the risk of birth defects or sheer difficulties in having children, though co-author Dr. Agustin Conde-Agudelo acknowledges that these reasons are more speculative.
This five-year gap reminded me of another aspect of family life: birth order effects. Frank Sulloway, in Born to Rebel, says that birth order effects are strongest for siblings who are fewer than five years apart. If there is more than a five-year gap between one sibling and the next it is as if you started over, and the next kid is more like another first born.
Putting these two things together, the Expert Advice of the moment would lead to this conclusion: Have your kids far enough apart that they can occupy distinct niches (unlike "Irish twins" born nine months apart). On the other hand, don't have them so far apart that they don't shape and influence one another.
I am happy to report that the Gruntled kids just about fall within the parameters: 17 months between the first two, then four years to the next. No birth weight of birth defect problems. And they definitely influence one another's roles, in the family and in the world.