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Out of OrderAre firstborns really smarter than their siblings?

By Emily Bazelon
Posted Tuesday, June 26, 2007, at 5:30 PM ET

Emily Bazelon was online June 28 to discuss this story. Read the transcript.

(Continued from page 1)

That could be especially true for families in which a child died in infancy. Maybe these deaths mean a higher rate of prenatal alcohol use or malnutrition or other factors that make these families different, Rodgers argues—and thus call into question the validity of comparing the younger brothers in them to younger brothers in other families. In Rodgers' work, using a 1990s American data set of about 2,500 children, birth-order differences in IQ melted away when he compared pairs of brothers within the same family instead of looking at the data across different families. The between-family vs. within-family split generally explains the contradictory results in the previous literature about birth order and intelligence: The studies that find an advantage for firstborns compare kids across families, and the ones that show no advantage compare pairs of kids within families. Rodgers thinks that what can look like a difference in IQ based on birth order and family size is really about parents with low IQs having larger numbers of children: "Have parents with lower IQs in the United States been making larger families? Yes. Do larger U.S. families make low-IQ children? No."

I'd like to stop there, but I can't, because Kristensen and Bjerkedal's report in Intelligence is a within-family analysis. And the IQ edge for the older brothers held when the data were sliced this way. Rodgers thinks that may be explained by a flaw in the authors' standardization of IQ scores. The potential problem involves what's called the Flynn effect—the tendency of average IQ to go up over time. It's possible that because of this effect, and the way that Kristensen and Bjerkedal did their IQ calculations, the results favored the older-born brothers. But Rodgers won't know until somebody reruns the whole analysis.

In the meantime, here's the issue that may matter more than anything else: A 3-point edge in IQ, as the New York Times pointed out in a sentence written to set off explosions of anxiety, could have "a cumulative effect that could mean the difference between admission to an elite private liberal-arts college and a less exclusive public one." Judith Harris points out that "more crucially, it could mean the difference between graduating from high school or not graduating, or between going to college and not going to college." Is there any evidence that younger siblings attain less educationally? To the contrary. Harris cited a 1989 study of more than 100,000 people, also published in Science, which found that in small and medium-sized families, birth order had no effect on how far kids go in school. (In really big families, the 7th- and 8th-born kids were more likely to continue their schooling, probably because their parents had more money by the time they showed up.)

Maybe in future studies, the IQ advantage for older siblings will again look like an illusion—my own fond hope. But if not, the younger ones appear to have figured out a way to make up for it. Three IQ points? Your kid brother or sister will see you that, and raise you an admissions ticket to college. If it exists, the smartness gap rolls right off them. Just like all those punches they took.

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Emily Bazelon is a Slate senior editor.
Illustration by Robert Neubecker.
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Remarks from the Fray:

Missing from the silly uproar over this study is any mention of Howard Gardner's work on the seven intelligences, despite vague allusions to such implications: older children might have higher IQs, but younger children tend to be more creatively successful; most of the staggering paradigm shifts in our society have been instigated by younger children. Not to mention that the IQ rule (or any other birth-order rule) is not hard and fast -- it is only predominant across an entire population (in Norway).

--Willialm

(To reply, click here.)

The study is an abuse of statistics, and few legitimate conclusions can be drawn. The IQ variance of 3 points in the mean doesn't give anyone an advantage - the difference between an IQ of 100 and 103 is no where near as significant as a difference between 118 and 121 (start of the end of the curve) or between 88 and 91 (start of the beginning of the curve).

If the study shows anything, it shows that some kids are a little more average than others.

It is also true that statistical information about groups cannot be legitimately applied to individuals within a group. In other words, while first borns as a group may have a mean IQ advantage, there is no reason to infer that any individual first born will have a higher IQ than any individual non first born child.

This isn't science, its conjecture. And, this conjecture is turned into pandering and gossip by the media. Frankly, if your IQ is 103, you stand a rat's chance in hades of getting into an Ivy League School. If it's possible to get into Harvard with an IQ of 103, its quite likely you would get in with an IQ of 100, because there is negligible difference between the scores.

Consider: if the IQ test has 100 questions, a 3 point difference on the statistical aggregate of the scores is less than 1 right answer difference on 40% of one thousand participants' tests.

--richard noggin

(To reply, click here.)

One of the problems with studying birth order effects - and the reason why our results may not match some older beliefs/studies - is that children are not raised together anymore. With so many kids in daycare, young children spend most of their waking hours in the care of others surrounded by children other than their siblings. Instead of developing with their siblings, they are surrounded by a group of similar aged peers. Even if the siblings are in the same daycare class, the effects are watered down due to the presence of many other same-aged children and being cared for by someone other than a parent.

I think what we will find now is a whole new effect. Children raised in daycare won't have the typical birth order effects. Eldest and only children won't have the same one-on-one treatment throughout the day and will have to learn to compete for attention, youngest children will have same-aged or younger peers to compare themselves to, and a host of other differences can occur when preschool kids are put into groups or classrooms all day long. Whether you are a first, middle, last, or only child, you will spend most of your day in a similar environment and therefore will develop in a very similar manner.

I'd like to see more information about the effects of being raised in a large group of non-related children by someone other than a parent, rather than trying to make sense of data that may no longer be applicable.

--playingtrix

(To reply, click here.)

Let me explain a bit about the way IQ works. Based on a large database, everyone's IQ is normalized, so that the mean (or average) is 100. Additionally, the standard deviation is set to be 15. A standard deviation is just a measure of how random people's IQs are. Let me explain that a bit more. We can expect that 68% of the population has an IQ within one standard deviation of the mean. 96% of the population is within 2 standard deviations of the mean. In other words, 68% of the population has an IQ between 85 and 115, and 96% have an IQ between 70 and 130.

A difference of 3 points, or one fifth of a standard deviation is a meaningless difference. I might expect that same difference depending on whether or not I had my morning cup of coffee before taking an IQ test.

That's not to say that there isn't a difference. I'm just saying that the difference is so small, its practically meaningless.

--KALMBOB

(To reply, click here.)

Bazelon only tangentially questioned the value of IQ itself. Why are we still measuring IQ, anyway? Although it might be useful to measure children's IQs, it makes no sense to do so for adults, since numerous other studies have shown little correlation between IQ and adult success.

The oddest thing about this study, to me, is that people like Bazelon are taking it so seriously. I sent this article to my younger sister as a joke, she took it in kind. That's exactly how it came up in front of my friends as well. Maybe we all lack competitiveness, but I doubt it. The truth is, most people realize that IQ means almost nothing in the real world. It's a purely academic measure, which may be why academics are still interested in it.

--AnikaG

(To reply, click here.)

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