Research procedures that focus on the history of a particular individual or group are called case studies.
Another widely cited case study is that of Phineas Gage, a railway construction foreman who suffered a bizarre accident in 1848, when a three-foot-long iron rod was driven into his skull. The iron rod entered beneath his left eye and exited through the top of his head, destroying much of the prefrontal cortex. Because Gage survived, he provided an opportunity to investigate the effects of brain damage on outward behavior. Although he could still speak and move normally, his friends reported a big change in his personality. The accident transformed him from an amiable, dependable worker into an inconsiderate, foulmouthed, boorish individual. This case, and others like it, suggested that parts of the frontal lobes control social judgment, decision-making and goal-setting.
Although findings from case studies can be valuable, the method has limitations. One major weakness is the size of the data sample. Because of the exclusive focus on a particular individual or group in a case study, the researcher has no way of knowing whether that individual is typical of people in general. Does the case of Phineas Gage indicate how everyone with a similar injury might be affected? The answer is no. No two people could ever suffer from identical injuries. Definitive statements about the relationship between brain damage and behavior can only be obtained by means of controlled investigative procedures. Moreover, case studies, by their very nature, do not permit the researcher to draw any conclusions as to causality. In a conventional experiment, the researcher begins with one or more discrete hypotheses that are tested by the controlled manipulation of specific variables. Case studies do not permit careful control, rendering it impossible to identify a specific causal association from the studies alone.
The difficulty of drawing causal inferences from individual case studies is further illustrated by the case of Genie Wiley, a 13-year-old girl who was seriously neglected by her parents for most of her childhood. From the age of 18 months she was confined to a small room and denied any opportunity for social interactions or normal human contact. No one spoke to her, and she was punished for making any sounds herself. This extreme case permitted researchers to test the hypothesis that there is a critical period of language acquisition. This hypothesis maintains that a child's ability to learn the native language effectively ends at the onset of puberty. Genie was 13 when she was rescued and she could not speak. Her condition was particularly interesting to scientists who researched language learning. Her only sounds were high-pitched whimpers. Placed in a nurturing environment, would Genie learn to speak? If so, the critical-period hypothesis could be refuted. After several years, Genie was able to use words to convey some of her needs, but her grammar and pronunciation remained abnormal and impoverished. While this case supports the critical-period hypothesis, crucial information is missing from the picture. It is not possible to ascertain whether Genie was born with cognitive deficits that might have prevented normal language development, even in the absence of her social isolation.
Because of their very narrow focus, case studies can sometimes be misleading. The case of David Reimer (1965–2004), called John/Joan in early media coverage, had to do with a normal baby boy whose penis was mutilated by a botched surgery. Raised initially as a girl and severely mishandled by psychologist John Money, Reimer came in his teen years to identify as male. His case was first argued as an illustration of how sexual identity is a social construct; it later proved differently when Reimer went public with the treatment he and his identical twin brother had experienced. Reimer committed suicide.
See also Gage, Phineas; Research methodology ; Retrospective cohort study ; Sampling methods .
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