I agree with mutt, That the spectrum of light will affect the male to female ratios but what I think plays the largest role in this process is the growers enviromental conditions.
what I mean by this is: all cannabis plants have an ideal enviroment that they can thrive in. The farther away from these conditions that we get the more males that will pop up. This is because a male plant can take more stress then a female. A male plant can produce QUALITY pollen even if it is very stressed. So in its early life if the plant feels that it does not have a good chance at becoming a female and producing seeds it will become male so that it can contribute to the community, by passing on its genes and pollinating a female.
In nature a seed geminates and pops under the spring sun, which is what MH simulates. It is also cool, not hot, and there is usually a spring breeze.
With all this said, I usually grow my seedlings under cool white flouros for the first 2-3 weeks and then move them under my HPS, With a nice sized fan blowing on them. I only own flouros and 2 - 1000 watt hps's and I always get a very nice ratios.
Although I have done a lot of research, I in no way am trying to pass this info off as facts, these are more just my methodologies and observations. but for me if I control the enviroment then I get more females.