[简答题]A large proportion of the studies of behavior used animals as subjects, especially pigeons, rats, and rabbits. There are a number of reasons why researchers in this field frequently choose to conduct their experiments with nonhuman subjects. First of all, the possibility of a placebo effect is minimized with animal subjects. 46)Whereas a human subject’’s behavior may be drastically altered by the knowledge that he or she is being observed, this is unlikely with animal subjects because most studies with animal subjects are conducted in such a way that the animal does not know its behavior is being monitored and recorded. Furthermore, it is unlikely that an animal subject will be motivated either to please or displease the experimenter, a motive that can ruin a study with human subjects. A second reason for using animal subjects is convenience. The species most commonly used as subjects are easy and inexpensive to care for, and animals of a specific age and sex can be obtained in any qua