You have all deviated from the question. these replies have in no way helped anyone who is in need of an academic research. you changed the question to rather, "WAS THE IGBOS PRACTISING KILLING OF TWINS?" The question is what was the reason. Believe it or not, we are all Africans and guilty of all African practices.

I think or I have read something concerning this before. I think the reason is that they believe that only animals give birth in pairs or multiples, like the case of a hen, goat, pig, etc who give birth to more than one offspring. So, the reason had it that they believed that it was an animal so to say, or that it is not a real human being and as such, abomination(to the land), that a human being should give birth to animals. I came across something like this in a book. I can't categorically say which book but THINGS FALL APART may give a clue.
Thanks.