A Time article last year initially sparked my interest. This article was based on a Norwegian scientific study conducted on 250,000 male soldiers and their IQ test scores. Amongst its many conclusions, the most powerful of which conceded that IQ decreased from the eldest child to the youngest. Though this might not be a surprise to us, as we've all grown up the stereotypes and assumptions of eldest child being the most responsible and clever, while the youngest is the most doted and perhaps not as bright. Nevertheless, as a man of science, it makes it all the more convincing to back up assumptions with scientific evidence.
The Time article explains the reasons for the disparity in intelligence amongst siblings. Is it because of the lack of resources as another child comes along?
The trends exist. In fact, tons of websites exist talking about birth order and some very obvious observations. The reasons for the IQ drop amongst siblings are also discussed in the TIME article which is definitely worth reading. If you're the eldest, then you're more likely to struggle through and try and figure out solutions to problems, whether it be Lego, or dealing with parents. The younger children on the other hand, already solutions at hand from observing the elders, and they are more likely to follow the lead. As a result their problem solving skills are weaker. It's not just following the lead, but younger children are constantly challenging elder ones throughout both their developmental years. This is because, apart from the parents, eldest children play important roles in answering their siblings questions and dealing with their curiosities. Since they play these 'teaching' roles, their IQ develops. Whatever, the case may be, birth order is very interesting and its no wonder that people read into birth order as religiously as they read into horoscopes for example.
While the eldest in an overpopulated brood has it relatively easy—getting 100% of the food the parents have available—things get stretched thinner when a second-born comes along. Later-borns put even more pressure on resources. Over time, everyone might be getting the same rations, but the firstborn still enjoys a caloric head start that might never be overcome.
Food is not the only resource. There's time and attention too and the emotional nourishment they provide. It's not for nothing that family scrapbooks are usually stuffed with pictures and report cards of the firstborn and successively fewer of the later-borns—and the later-borns notice it. Educational opportunities can be unevenly shared too, particularly in families that can afford the tuition bills of only one child. Catherine Salmon, an assistant professor of psychology at the University of Redlands in Redlands, Calif., laments that even today she finds it hard to collect enough subjects for birth-order studies from the student body alone, since the campus population is typically overweighted with eldest sibs. "Families invest a lot in the firstborn," she says.
Oh, and if anyone is wondering, I am the eldest of two siblings...