if you lower income tax it has a positive effect on economy because it means that more money will be moving around. As long as money is switching owners (instead of sitting on someones bank account) a countries gdp will continue to rise.
Obviously it can also have some negative effects, such as lowering the amount of money that the government gets etc. It all depends on how much you raise/lower the tax and when. A lot of factors involved and some of them are quite damn confusing.
(at least to me) And the thing is that even though the experts tell whats the best thing to do when thinking about the future of economy in a certain country the politicians never listen.