I wonder how much of this was the result of food scraps from US bases? I've been told a number of Korean dishes use ingredients that were begged from American GIs during and after the war. Things like hotdogs and american cheese that then got used creatively. I know other species will see a huge birth spike from discovered food sources. But I don't know if human's are smart enough, or have expensive enough pregnancies and child rearing to avoid this potential trap.