First-Borns and IQs

I'm not surprised by the news that first-borns apparently have IQs that are a couple of points higher than later-borns. Just the difference in parental environment would do the trick, I think. With my own first-born, we had plenty of time to play with him, read to him, talk with him, and so forth. He was the only kid around, and he got 100% of our attention. Thus, for example, he could identify the entire alphabet at 18 months. But our youngest two children have been about a year behind. Instead of 100% of the parental attention, they each get 20% (give or take). There's just not as much time to read the alphabet book with the youngest, when the seven-year-old wants to show you whatever book he's reading.

I have noticed, however, that the youngest children have been able to pick up quite a vocabulary from the older kids, one that's quite different from the vocabulary our oldest had when he was two years old. For instance, "I tell Mom!," or "Get out of here," or "Stop copying me."