A severe problem for evolution theory is to explain the evolution of sexual reproduction and behavior. The theories proposed include dual evolution, i.e., the sexes evolved separately but greatly influenced each other’s evolution. Another major theory is sexual selection, a process by which organisms directly influence each other’s evolution. The creationist model proposes that the sexes were designed to be physically and mentally compatible with each other, and for humans a harmonious physical and mental relationship will result if the couple lives in harmony with the physical and mental constraints of this design.