In one of the oddest theories about intelligence, Robert Zajonc argued that birth order was the single strongest determinant of a person's IQ. Firstborn children, he argued, are likely to be smarter than later-born children. At first blush, this seems almost silly. How could your position within a family explain anything about your intelligence? Do a woman's eggs get weaker as she produces more babies? But Zajonc's explanation for his prediction makes some sense and fits with other data. Zajonc argued that the firstborn is likely to have the highest IQ within a family because he or she benefits from a richer intellectual environment than subsequent children. Imagine, he argued, that the average combined IQ of two adult parents is 200. The firstborn child received all the benefit of that combined IQ. But the next child, and those who come after, have to share that intellectual environment. So the firstborn gets to grow up in a 200 IQ environment, while the later-born children are developing in a 100 IQ environment (200, the parents' IQ score, divided by two children). The formula is so simple it's almost hokey. But the logic behind it is backed up by other research. It is not that somehow the parental genes for intelligence get thinner or weaker with each child. Instead, each child, in theory, experiences a more diluted intellectual environment. Think of it in terms of any big family you know. The parents probably talked quite a bit to their firstborn, discussing what their little boy could see out the window, answering his questions about why his milk turned pink when he ate Lucky Charms, and asking him what was going to happen to the ice cube if he left it lying on the floor. But the third or fourth child in a busy household is much less likely to hear and be part of those kinds of exchanges. So, in fact, later-born children grow up in a somewhat weaker intellectual environment.
Interestingly enough, Zajonc's formula predicts that a child who is born a long time after the last one benefits from his position in the family. Why would this be so? Because if you have three much older siblings, your intellectual environment reflects the IQ of five grown-ups, not two diluted ones. Your much older brother and sister talk to you the way an adult would.
Zajonc's research is so parsimonious it is hard to accept. Everyone can think of exceptions—the twenty-four-year-old who is brighter than her twenty-five-year-old brother, the one born eight years later who is not as bright as his three much older siblings, and so on. But the logic behind his formula fits perfectly with all of the data showing that children benefit from conversation. When parents use talk as a way of reflecting on and making sense of the world around them, a child's intelligence benefits. And the data suggest that families with economic resources engage in more of this kind of conversation. So being rich does not make you smart, but having more wealth might be tied to having more conversation, which contributes significantly to a child's intelligence. In other words, the bank account itself does not explain wealthier children's advantage in school—what the bank account provides explains it.