My Immature Brain Made Me Do It?

On November 9th, 2009 the Supreme Court will hear arguments on whether the 8th amendment's ban on cruel and unusual punishment prohibits courts from sentencing children to life without the possibility of parole for the commission of a non-homicide. Joe Sullivan and Terrance Graham were 13 and 17 years old respectively at the time of their crimes. Although their crimes were violent, no one was killed. Yet they were both sentenced to life without parole by Florida judges. The question before the Court is whether the 8th amendment prevents courts from locking up children and throwing away the key.

Common standards of decency and the fact that adults rarely get life without parole for the same crimes make a strong case for the petitioners. But Sullivan and Graham are not really the ones on trial. The elephant in the room, and the thing that the Court has taken deliberate steps to leave out of its rulings in the past, is the human brain. The last time around, in Roper V. Simmons (2005), the Court held that it is unconstitutional to execute someone for committing a crime while under the age of 18. Despite numerous briefs submitted by mental health advocacy groups suggesting that the brain is not fully mature until the mid-20's, the Court wisely did not use this information in its ruling.

Come November, the Court should once again ignore the growing drumbeat to blame the immature brain. Hopefully, the Court will leave neuroscience out of its decision.

When you take an MRI of a person's brain, it is easy to distinguish gray and white matter. A wealth of neuroscience data has shown that these two parts of the brain change during development from childhood to adulthood. The density of gray matter appears to decline through adolescence in a process known as pruning. Although not well understood, it is believed that pruning allows the brain to function more efficiently. White matter, which contains the connections between neurons, gets its name from a waxy substance called myelin. During adolescence, the myelin becomes gets denser and white matter more organized, which speeds up information transmission throughout the brain.

These developmental changes occur at different rates in different parts of the brain. Everyone has heard by now that the frontal lobes are the last parts of the brain to mature, not reaching their adult form until the mid-20s. It is precisely this observation that many have seized upon as an explanation for why teenagers make bad decisions. The immature brain has been offered as an explanation for why teenagers are more impulsive and why they are more sensation seeking than adults. According to briefs submitted to the Court, the immature brain means that adolescents are less responsible for their actions as adults (ABA briefs here).

But there are serious flaws with the "immature brain made me do it" argument. In fact, my group recently published a study calling this argument into question (PLoS One, 2009, or the Scientific American article about it). All of the neuroscience findings cited in the briefs rely on a correlation of brain structure with either age or a measurement of cognitive function. Correlation means that you take one measurement and see how it changes with some other measurement. While on average, these conclusions are statistically valid, there is too much variation from one person to another to draw conclusions about any one individual. But you won't find individual variability mentioned in any of these briefs.

In fact, any measurement we can make on the brain can only account, at most, for 30% of the differences in a particular behavior from one person to another. This means that brain data cannot account for 70% of what people do. The correlations with age are no better. It's really no different than the growth charts that pediatricians use. These charts show that children get taller as they get older. But try using a growth chart in reverse. Could you determine a child's age from their height? Would you be willing to stake the child's life on that estimate?

The brain data cut both ways. If the Court invokes neuroscience, it opens a floodgate for MRIs in the judicial system. It would become necessary to perform MRIs on every child defendant to determine if their brains were sufficiently mature to stand trial, or be punished, as an adult. What should be done with a child who has an unusually mature brain for their age? Or an adult with an immature-looking brain?

And then there is the problem of the aged adult. By age 70, most people will have lost about 8% of their youthful gray and white matter. Using the same logic, it would be considered cruel and unusual punishment to sentence older adults to life without parole. Anyone for an 8th amendment appeal by Bernie Madoff?