Older kids not actually smarter than their younger siblings

I really don't know what the mechanism for older children being smarter than their younger offspring would be but whatever it was this research debunks it.
"The findings contradict many studies over the years that had reported that older children are generally smarter than their younger siblings. This new study, based on a large, nationwide sample, suggests a critical flaw in that previous research, said Aaron Wichman, lead author of the new study and a teaching fellow in psychology at Ohio State University."