However, if you did wish to evaluate such a proposition, then you'd need to randomly sample persons having birth dates from within each of the 12 zodiac signs, match them on relevant characteristics (age., sex, profession, level of education, etc.) then collect data as to their performance on some measure of intellectual functioning that folks consider a valid indicator.
This being smart when born in September has to do with the school start. The school start in the autumn in our country. "Researchers found that "being relatively old for grade (being born in September versus August)" was more likely to result in better grades. This seems to be supported by the fact that those born in August — and are 11 months younger — were most likely to have the lowest grades." The brain was less developed! It has nothing to do with horoscopes.
I believe that intelligence is transmitted through the genes of the DNA and is patient and developed according to the environment and upbringing that the individual receives, and the order of the constellations is just an illusion