Nature Versus Nurture
https://www.theatlantic.com/business/archive/2013/10/first-children-are-smarter-but-why/280725/ In the study noted in the article above, investigators confirmed the postulation that first-children generally excel beyond the levels achieved by their younger siblings, and they asked “Why?” A genetic basis for the difference (i.e., where some advantage would be conferred on first-borns by “Nature” … like the first-born gets the best genes) was…