One of the major disadvantages to animal testing is that a significant number of animals are harmed or die as a result of experiments and testing. Unfortunately, many of the substances used on animal subjects never receive approval for human use or consumption. Those who oppose animal testing consider this a very important point, because humans receive no direct benefits as a result of the deaths of these animals. Opponents also argue that animals are dissimilar enough from humans to make the results of animal tests unreliable. A related criticism is that testing induces stress in the animals, meaning that the subjects do not react to experimental substances in the same way that they might in more natural circumstances, making the results of experiments less valid.
By 1988, in response to raids by ALF and other groups, more than 20 states had enacted protective legislation prohibiting interference with animal research and agricultural facilities. In August 1992, citing the inability of state and local law enforcement agencies to conduct interstate or international investigations, Congress passed comparable federal legislation. The Animal Enterprise Protection Act of 1992 (18 . § 43) prohibits the disruption of "animal enterprises" such as research facilities and zoos by intentionally stealing or damaging property including animals or records.