Analyze each of the following reports, explaining the ways in which the method of difference has been applied in the investigations recounted. Discuss the strengths and weaknesses of the method of difference as it is used in each case.

To determine the role of specific genes, mice are bred in wahich certain genes have been deleted. Such mice are called “knockout mice.” When normal mice are placed in a lighted room, with dark corners, they go immediately to the dark. In one recent experiment the mice, upon entering the dark, encounter a mild electric shock, and very quickly learn to stay away from those dark regions. Mice who lack a gene called Ras- GRF learn to be wary just as quickly as do normal mice. But, unlike normal mice, the knockout mice throw caution to the winds the next day, and chance the dark corners again and yet again. It appears that the Ras-GRF gene—probably very much like the analogous gene in humans—plays a critical role in the ability of the mice to remember fear. This gene is almost certainly crucial for the survival of mammals.
—Reported in Nature, December 1997