Seems plausible, even self evident.
The first gets more attention, more doting, prodding, stimulation, until the next comes along.
The next one gets—well I wouldn’t say half— but less.
The eldest resolves his/her jealousy by assuming authority and responsibility over the younger interlopers.
That sort of thing might influence IQ tests, whatever they are.
Kerry's Birth Order Was An Early Predictor Of Outcome Of Presidential Race for this Psychologist
PR9.NET November 22, 2004 - Tucson, Arizona – In a press release dated February 13 of this year, Dr. Kevin Leman predicted that all else being equal, Senator John Kerry may not have what it takes to win the presidency, for no other reason that that Kerry is a second-born "middle" child.Leman suggested that critical issues may pop up in Kerry's campaign should he maintain too much control of it. Recent reports by embedded reporters in the Kerry campaign suggest his prediction to be true--that indecision and disorganization in the campaign could have cost Kerry his bid for the presidency. After all, according to Leman, as a middle-born Kerry would tend to be sociable, friendly, and outgoing, but also want to please everyone with every decision, thereby leading to indecision and "flip-flopping" (a term used by the Bush campaign to describe Kerry).