Yay! Babies! (... uh ... or not)

Statistics can lead to wrong conclusions. Very very wrong. Here's an interesting example proving that...

From Jakob Nielsen: Studies show that intelligence declines by birth order. In other words, a person who was a first-born child will on average have a higher IQ than someone who was born second. Third-, fourth-, fifth-born children and so on have progressively lower average IQs. This data seems to present a clear warning to prospective parents: Don't have too many kids, or they'll come out increasingly stupid. Not so.

There's a hidden third variable at play: smarter parents tend to have fewer children. When you want to measure the average IQ of first-born children, you sample the offspring of all parents, regardless of how many kids they have. But when you measure the average IQ of fifth-born children, you're obviously sampling only the offspring of parents who have five or more kids. There will thus be a bigger percentage of low-IQ children in the latter sample, giving us the true -- but misleading -- conclusion that fifth-born children have lower average IQs than first-born children.

Any given couple can have as many children as they want, and their younger children are unlikely to be significantly less intelligent than their older ones.