Biological sex interacts with environment in ways not fully understood.[33] Identical twin girls separated at birth and reunited decades later have shown both startling similarities and differences.[34] In 2005 Kim Wallen of Emory University noted, "I think the 'nature versus nurture' question is not meaningful, because it treats them as independent factors, whereas in fact everything is nature and nurture." Wallen said gender differences emerge very early and come about through an underlying preference males and females have for their chosen activities.