About 2,400 years ago, a Greek physician writing in the tradition of Hippocrates took on a disease that people called "sacred." Epileptic seizures were widely blamed on gods or spirits. The author of the treatise On the Sacred Disease disagreed, arguing that epilepsy has natural causes in the brain like any other illness. He went further: "Men ought to know that from the brain, and from the brain only, arise our pleasures, joys, laughter and jests, as well as our sorrows, pains, griefs and tears." Modern neuroscience is, in a sense, a long confirmation of that claim, and it brings new responsibilities.

When the brain goes wrong

Epilepsy affects around 50 million people worldwide, according to the World Health Organization. A seizure is a burst of abnormal, synchronized electrical activity in which large groups of neurons fire together, like a stadium crowd chanting in unison when it should be holding many conversations. Medicines that strengthen inhibition or calm overactive channels control seizures for most patients, and stigma remains one of the condition's heaviest burdens in many countries.

Stroke, the interruption of blood supply to part of the brain, is a leading cause of death and disability worldwide, as the third level showed. Alzheimer's disease is the most common cause of dementia. In 1906 the German psychiatrist Alois Alzheimer described the brain of a patient named Auguste Deter, who had suffered severe memory loss and confusion in her fifties. He found the two features still used to define the disease: clumps of amyloid protein, called plaques, and twisted tangles of tau protein. For decades, drugs aimed at amyloid failed. In 2023 the US Food and Drug Administration gave full approval to lecanemab, which modestly slowed decline in people with early Alzheimer's, by about a quarter over 18 months in its main trial, though it carries risks of brain swelling and bleeding and its practical benefit is debated.

Some brain disease is caused by how we play. In 2005 the pathologist Bennet Omalu reported abnormal tau deposits in the brain of the former American football player Mike Webster. The condition, chronic traumatic encephalopathy or CTE, is linked to repeated blows to the head over many years, not only to obvious concussions. At present it can be confirmed only by examining the brain after death.

Mental privacy

Neuroscience can now read some information from the brain. Implanted devices have let paralyzed people move computer cursors, type and even produce synthesized speech by decoding signals from the motor cortex. Consumer headbands record EEG to track focus or sleep, and the data they collect may reveal more than users expect. Many scholars argue that the mind deserves special protection, because it is the last private space and the source of the freedom of thought that underlies every other right. In 2021 Chile became the first country to write protection of brain activity and the information from it into its constitution, and in 2024 Colorado became the first US state to add neural data to its privacy law.

Neuroscience in the courtroom

Brain evidence increasingly appears in courts. Companies have marketed fMRI lie detection, but in a 2012 case, United States v. Semrau, federal courts found fMRI lie detection too unreliable to admit as evidence. Scans are more often offered at sentencing, to argue that a defendant's brain injury or disorder reduced their control. A famous 2003 report described a Virginia man who developed sudden pedophilic urges; doctors found a large tumor in his frontal lobe, the urges faded when it was removed and returned when it grew back.

Cases like that are rare. The deeper problem is that a brain pattern found on average in a group may not apply to any one person. Many people with unusual scans never commit crimes, and many offenders have ordinary scans. There is also a question of dignity. Explaining behavior through the brain should not turn a person into a malfunctioning machine, either to excuse everything or to justify locking people away for what their scans predict. It also raises old philosophical questions about free will.

Worked example: the scan at sentencing

A man is convicted of assault. His lawyer presents a scan showing reduced activity in his prefrontal cortex and argues for a lighter sentence. How should a judge weigh it?

First, the methods of this course apply: was the scan compared against a fair control group, and is the reduced activity a known, reliable finding or a reverse inference? Second, the group to individual problem: even if people with violent records show this pattern on average, that does not prove it caused this act. Third, does the evidence show he could not control himself, or only that control was harder? The law usually asks the second question at sentencing and the first at trial. Finally, the evidence cuts both ways: a brain that makes violence more likely could argue for mercy or for longer detention. A judge who understands neuroscience will treat the scan as one piece of evidence about a whole person, not as a verdict written in the brain.