Can We Stop Talking About Birth Order and Birth Spacing? (Or Put Another Way, Just Enjoy Your Kids)

Parents have lots of things on their mind when they are thinking about having a second child. If I could deliver one message to them, it would be to stop worrying about the effects of birth order and birth spacing on development. Why? Simply put, the statistical effects are so small that they are of no practical value to any parent. 

Here’s a case in point. I just read this post in the NY Times Motherlode column, which is very well-written. It refers to yet another study on birth spacing, with the idea being trying to come up with an optimal space between siblings to promote cognitive development. The research was well done (it analyzes data collected from a large and informative longitudinal study) – I certainly don’t have any issues with it. The analyses were sophisticated. The inferences drawn are in line with the analyses. But I’m not going to get into the details of the findings. Wanna know why?

The statistical effects of birth spacing are minuscule. They are significant because the sample size is very large – large sample sizes can detect even the smallest of statistical effects (this is a basic principle of statistics). Back when I was in training, I had the pleasure of being exposed to the teachings of a number of eminent statisticians, who always cautioned us to keep this principle in mind – statistical significance doesn’t imply anything about the magnitude of the effect.

As a matter of fact, the researchers should be commended because they reported the effect sizes in their paper, which allows us to interpret how influential birth spacing really is. And they don’t overstate the size of the effects when they discuss their findings. They are actually delivering good science which tells us that birth spacing is really a negligible influence on development.

So where I deviate is in drawing a take-home message for parents. I really don’t think there is one. Simply put, the spacing between siblings will statistically have an extremely small effect on cognitive development.  So if I had my way, we’d stop delivering the message that you should systematically think about how to space your kids  - and along with that refrain from making you believe that their birth order will have a huge impact on their lives. Sometimes the take-home message from science is that there really isn’t a big one after all.

Image: Girl kissing her little brother via Shutterstock

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  1. [...] been doing research on siblings for, well, a lot of years now). I’ve always felt that birth order was background noise – with a little signal – that got way too much attention in the popular press as [...]