I'm interested to know how intelligence would help to drastically reduce population. I assume you don't mean conceiving and executing schemes of mass extermination.
For the intelligent to avoid having offspring of their own is very feasible, of course. The problem would be getting everyone on earth to do the same. How would you set about it?
I went out to one of those sites that calculates your footprint on earth, and then tells you how many planet Earths we would need if everybody lived that way, and I thought of a new take on Population control.

First, everybody is required to fill out the questions to determine their footprint - not so much how they live, but how they 'pledge' or 'promise' or 'contract' to live in the 'new world'.  After you make your contract agreeing to how you will live, it figures out your expected foot print.  From there you are entered into the 'lottery', with your odds of 'winning' the lottery equal to 1:<your footprint>.  If it is determined that you will live with in a foot print that is sustainable you automatically win.  If you want to live a lifestyle that is 1:30, well, only 1 in 30 will win, and the others are made into fertilizer.  If you break your contract, you have the same fate.

Kevin