Some Preliminary Thoughts on a Biological Explanation for the Age Crime Curve

This working paper presents preliminary theoretical thoughts about a biological explanation for the age crime curve, prepared in connection with the American Society of Criminology annual meeting in 2012. The ideas developed here were later presented at the American Society of Criminology annual meeting in 2013 as Arnold (2013), Testing Quetelet's Biological Explanation for the Age Crime Curve, and more fully elaborated in Arnold (2016), The Criminological Puzzle. The age crime curve — the well documented population level pattern of rising criminal activity in adolescence followed by decline in adulthood — was first identified by Quetelet (1831/1984) and has remained without a satisfactory causal explanation despite nearly two centuries of criminological inquiry. This paper argues that the age crime curve must be explained using population parameters and biological factors, and that no single biological factor is sufficient to account for the distinctive shape of the curve. A two factor four phase biological model is proposed in which the interaction between developmental trajectories of physical strength and brain capacity explains the age crime curve. Physical strength has a direct positive relationship with crime — as strength increases in adolescence, the physical capacity to commit crimes increases. Brain capacity has an inverse relationship with crime — as decision making capacity develops, the propensity for rule breaking behavior decreases. The age crime curve emerges from the interaction of these two trajectories operating at different developmental rates across the life course, with a critical threshold below which rule breaking behaviors are not recognized as crimes. A hypothetical co-plot of strength and rule breaking trajectories produces a curve that closely matches the actual age crime curve from the Uniform Crime Reports. The model is evaluated against eight requirements that any adequate theory of the age crime curve must satisfy, including the requirement that the explanation use population parameters, account for analogous behaviors such as aggression and rule breaking, and provide a parsimonious multi-factor solution. The two factor biological model satisfies all eight requirements. The data fit the model almost perfectly, suggesting that any competing theory would need to provide a better empirical fit or more logical assumptions. This paper is archived to preserve the intellectual timeline of the theoretical development leading to Arnold (2016), The Criminological Puzzle. It builds on the theoretical groundwork established in Arnold (2008), Agency in the Life Course of Criminal Offenders, and connects to the broader empirical framework developed in Arnold (2009), The Nonlinear Dynamics of Criminal Behavior, and the broader theoretical framework of The Physics of Living Systems.