Hey, don't get all bent out of shape about it. It's just a fact -- at least according to Norwegian scientists -- that older siblings tend to have IQs about 2.3 points higher than their junior sibling. Likewise, that junior sibling, should he or she have an even-more-junior sibling, is probably a bit smarter than that person. Turns out that IQ tests are mandatory for people entering the military in Norway, so statisticians had hundreds of thousands of young people's IQ tests to sort through, and came up with this little theory (published in New Scientist). So what's the reason? It's not genetic, it's social:
Older siblings might have a higher IQ because they act as surrogate parents from time to time, tutoring their younger siblings in certain situations. Another possible explanation is that parents have more time and resources to invest in their firstborn child. As a result of this, the eldest child usually has the most one-on-one time with its parents, and receives greater exposure to their sophisticated vocabulary.
They also note that second-born children whose older sibling(s) died early in childhood scored on par with other firstborn kids; on average, all were 13% more likely to be of "above-average" intelligence than their younger siblings. Now, I'm sure some of you will disagree based on personal experience, and I think I speak for everyone when I say, we'd love to hear some stories about dumb things your older brother/sister has done. Let's see if we can turn this thing around!