Most of the twins are not always identical or even share the same gender, which has remained a longstanding evolutionary puzzle. Identical twins result when a single fertilized egg accidentally splits in two, whereas, fraternal twins result when two eggs are released and fertilized. Recently researchers used computer simulations and modeling to shed light on why natural selection prefers releasing two eggs, despite the low survival rates of twins and the risks of twin births for mothers. The modeling indicates the ideal strategy would be to always double ovulate but never produce twins, so fraternal twins are an accidental side effect of a profitable strategy of double ovulating.