XYY Chromosomal Abnormality Defense

A legal theory that holds that a defendant's XYY chromosomal abnormality is a condition that should relieve him or her of legal responsibility for his or her criminal act.

Criminologists have examined many theories as to why a person becomes a criminal. Since the nineteenth century, biological theories have been proposed that seek to link criminal behavior with innate characteristics, yet these theories have been strongly challenged by the scientific community. With the development of modern genetics, scientists have noted abnormalities in the chromosomal structure of some people.

A chromosome is the threadlike part of the cell that carries hereditary information in the form of genes. The normal human genetic complement consists of 23 pairs of chromosomes. One of these pairs determines sex. Women have two X chromosomes and men usually have an X and a Y chromosome. However, in 1 in 500 to 1,000 live male births, an individual has an extra Y chromosome. This XYY abnormality is often characterized by tallness and severe acne and sometimes by skeletal malformations and mental deficiency.

With the discovery of the XYY abnormality in 1961, some social scientists proposed a link between the abnormality and aggressive and impulsive behavior. This "supermale" syndrome seemed confirmed when studies of prison populations showed the presence of the abnormality to be significantly higher than in the general population.

Armed with these studies, defense attorneys sought to use the XYY chromosomal abnormality as a criminal defense theory. However, the defense has never been successfully used in the United States. Though the abnormality can be easily diagnosed using a blood test, the courts have rejected the defense because of the lack of conclusiveness of SCIENTIFIC EVIDENCE regarding the theory of criminality.

The legal community's misgivings have been confirmed by subsequent studies of the general population, especially those in which affected individuals were observed from early childhood over a long period of time. These studies have cast serious doubt on the validity of linking the chromosomal anomaly directly to behavioral abnormalities. Numerous XYY individuals live normal lives as law-abiding citizens.