obviously the neglect & killing of babies has as strong effect on this, though i also found when i looked it up that asiatic nations have the slight tilt at birth as compared to african nations toward males (european, middle east & south asians were in the middle). but in the case of a place like pakistan, i have read that reluctance to admit or show female family members in purdah also leads to undercounts (this might apply to a lesser extent in patriarchal northern india too).