完形填空 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 quantities the experimenter needs. 47)Once animal subjects are obtained, their participation is as regular as the experimenter''s--animal subjects never fail to show up for their appointments, which is unfortunately not the case with human subjects. 　　48) Probably the biggest advantage of domesticated animal subjects is that their environment can be controlled to a much greater extent than is possible with either wild animals or human subjects. This is especially important in experiments on learning, where previous experience can have a large effect on a subject''s performance in a new learning situation. Likewise, if a human subject tries to solve some mystery as part of a learning experiment, the experimenter cannot be sure how many similar problems the subject has encountered in his lifetime. 49) When animals are bred and raised in the laboratory, however, their environments can be constructed to make sure that they have no contact with objects or events similar to those they will encounter in the experiment. 　　A final reason for using animal subjects is that of comparative simplicity. 50)Just as a child trying to learn electricity is better off starting with a flashlight than a radio, researchers may have a better chance of discovering the basic principles of learning by examining creatures that are less intelligent and less complex than human beings The assumption here is that although human beings differ from other animals in some respects, they are also similar in some respects, and it is these commonalities that can be investigated with animal subjects.