Every human being, after the first few days of their life, is a product of two factors; on the one hand, there is their congenital endowment; on the other, there is the effect of environment, including environment.
Is the use of their here grammatical?
Please give your views.
vsureshIs the use of their here grammatical?
Yes. It is quite common to use they, them, their, theirs, and themselves as a non-gendered singular pronoun.
By the way, unrelated to your question, I couldn't help notice that there's something wrong here:
vsuresh the effect of environment, including environment.