decision making must include the examination of social and emotional influences on these cognitive abilities (Scott et al., 1995; Steinberg and Cauffman, 1996; Piquero et al., 2011).

Adolescent Brain Development

The last decade has provided evidence of significant changes in brain structure and function during adolescence with a strong consensus among neuroscientists about the nature of these changes (Steinberg, 2009). Much of this work has resulted from advances in magnetic resonance imaging (MRI) techniques that provide the opportunity to safely track the development of brain structure, brain function, and brain connectivity in humans. Consistent with the previously described behavioral findings that adolescents have poor self-control, are easily influenced by their peers, and do not think through the consequences of some of their actions, the brain imaging findings strongly suggest that adolescents lack these abilities because of biological immaturity of the brain.

Structural Brain Development

Several studies have used MRI to map the developmental time course of the structural changes in the normal brain. Even though the brain reaches approximately 90 percent of its adult size by age 6, the gray and white matter subcomponents of the brain continue to undergo dynamic changes throughout adolescence and well into young adulthood. Data from longitudinal MRI studies indicate that increases in white matter are linear and continue well into young adulthood, whereas gray matter volume shows an inverted U-shaped course, first increasing and then decreasing during adolescence (Sowell et al., 2003, 2004; Giedd, 2004; Gogtay et al., 2004). These changes do not occur uniformly across development, but rather there are regional differences in the brain’s development (Thompson and Nelson, 2001; Amso and Casey, 2006; Casey et al., 2010). In general, regions that involve primary functions, such as motor and sensory systems, mature earliest compared with brain regions that integrate these primary functions for goal-directed behavior (Gogtay et al., 2004; Sowell et al., 2004). Similar to sensorimotor regions, subcortical regions involved in novelty and emotions (e.g., striatum, amygdala) mature before the control region of the brain and show greater changes in males than in females during adolescence (Caviness et al., 1996; Giedd et al., 1996a, 1996b; Reiss et al., 1996; Sowell et al., 1999). These developmental and gender findings are important in the context of this report, given the increase in criminal behavior during the period of adolescence, especially in males (Steffensmeier et al., 2005).

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