I agree with the previous poster. How does one quantify "smartness." I am assuming that you are referring to the cut-off date for when a child is able to start school. Are those kids who start school at an earlier age smarter? Why did they start school earlier, because they were smarter in pre-school, or simply because they could.
From the studies I have seen, and using induction, I do not think that anyone has ever been able to show correlation between the time of the year in which you were born and intelligence.
I would suggest getting insanely more specific, and also picking something that you are certain can be relatively easily quantified, i.e., something other than "smartness." Maybe use the score on one very specific type of test.