Human Biology and Criminal Responsibility: Free Will or Free Ride?

58 Pages Posted: 17 Apr 2009

Date Written: April 17, 2009


This article examines the role of the possible biological deficiency defenses in the criminal law. The topic is inspired by the renewed interest in biological and genetic research on behavior and the possible use of this research in a variety of criminal defenses in the United States and other countries. In general, the article presents three major arguments concerning biological deficiency defenses, using, respectively, a critique of biosocial science research, a statistical model of biological and sociological data, and an examination of theories and philosophies on causation and behavior.

The first of this article’s arguments states that there should be no defense to mitigate criminal responsibility except in the less that one percent of cases eligible for the insanity defense. Mitigating factors may be considered at the sentencing stage, not for determining the length of the sentence, but only to determine the type of facility for detaining or treatment of a convicted defendant. The second argument contends that social science research has not successfully demonstrated sufficiently strong links between biological factors and criminal behavior to warrant major consideration in determining criminal responsibility. This conclusion is based upon the results of one of this country's largest studies of the biological and sociological development of individuals from the time of their birth to young adulthood. Social science research, however, can be valuable in other contexts, such as predicting bias in death penalty sentencing, in which measures are better defined and there is a lesser burden of statistical proof. The article’s third argument suggests that there is no strong evidence to support either a strictly free will or a strictly deterministic philosophy in the criminal law regarding either the causes of crime or the determinants of criminal responsibility. Moreover, the notion of ‘cause’ has varying implications depending on the context. Statistical models of biosociological research support a philosophy of ‘degree determinism,’ however, that spans a lifetime. The criminal law should reflect this philosophy rather than a free will fiction.

The article has four Parts. Part I outlines some selected theories and research on genetic, biological, sociological, and environmental influences on criminal behavior. Criminal law cases and defenses that have used this research are discussed and criticized. Likewise, many of these theories are tested together in Part II, which examines research results from a longitudinal study of juvenile and adult crime in order to assess the rationale and desirability of a biological deficiency defense. This article then applies the results of this study's statistical model to develop a probability theory of behavior, which is discussed in Part III in the context of arguments supporting both free will and deterministic notions of criminal responsibility. Part III argues that a theory of ‘degree determinism’ more accurately reflects what have previously been called cause-and-effect relationships. Part III also assesses the feasibility of a biological deficiency defense given the problems posed by other proposed criminal defenses, such as Vietnam Stress Syndrome. Part IV concludes with a commentary on the appropriate and inappropriate uses of social science research and its relation to the goals and philosophy of the criminal justice system.

Keywords: Biology, Criminal responsibility, Delinquency, Human genetics, Insanity defense, Determinism, Degree determinism, Free will, Intellectual ability, Diminished capacity, Mens rea, Social science

Suggested Citation

Denno, Deborah W., Human Biology and Criminal Responsibility: Free Will or Free Ride? (April 17, 2009). University of Pennsylvania Law Review, Vol. 137, pp. 615-671, 1988, Available at SSRN:

Deborah W. Denno (Contact Author)

Fordham University School of Law ( email )

Fordham University School of Law
150 West 62nd Street
New York, NY 10023
United States
212-636-6868 (Phone)
212-636-6899 (Fax)

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