This "nature versus nurture" controversy polarized animal behaviorists during the 1950s. Ethologists criticized psychologists for using highly artificial experiments and for studying only a few species of animals: rats, pigeons, chickens, and primates. What could be learned about natural behavior from timing rats as they ran through mazes? Psychologists criticized ethologists for ignoring the development of behavior as young animals grew into adults. How could ethologists be so sure that learning did not occur, even in many simple animals?