The people who believe in decreasing the population should begin with themselves. That way they have solved the problem. And that problem is people who believe in the overpopulation myth.
Yes, we just hit 8 billion worldwide but we still can feed ourselves. The countries where starvation exists are run by tyrannical governments and it's those parasitical governments who are causing starvation, not overpopulation.
There's still a whole lot of land were you can go for days, weeks, months and years without seeing another person.