Stress is anything that causes the plant to have to adjust to its environment. Examples, drastic changes in lighting, drastic soil change(bad transplant), mostly things that you change that it has to adjust to will cause stress. Which will greatly incline a male outcome. Less change, better chance of female, more change, youll get a male.

In outdoor crops, because you cant control some of the changes they have to endure, males tend to be the most populant. In indoor crops, you can keep things a lot more steady and calm for them, basically playing God, thus increasing the female outcome...