Home > West's Encyclopedia of American Law > 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...