It's all in the matter of whether the child is given the position of responsibility or the amount of attention given or in short, the status given to the child.
Coming from a family of five children, and coincidentally favoured by my grandparents and thus singled out, I was the one given the most attention and hence more tutoring in my formative years, though I am the 4th child. As a result I am relatively smarter than my other siblings. My position as the one with the greatest capacity to give my family a better living, which came to me at an early age of 14 when I received a scholarship to study abroad, have a better education and a much better future, cemented my intellectual improvement with all the encouragement I got from those people who are hoping for my help in the future.
To me, and to my friends who are on the same boat, the relative IQ of a kid depends very much on the relative encouragement and importance given to him.
