Criminal courts in the United States are facing a surge in the number of defendants arguing that their brains were to blame for their crimes and relying on questionable scans and other controversial, unproven neuroscience, a legal expert who has advised the president has warned.

Nita Farahany, a professor of law who sits on Barack Obama's bioethics advisory panel, told a Society for Neuroscience meeting in San Diego that those on trial were mounting ever more sophisticated defences that drew on neurological evidence in an effort to show they were not fully responsible for murderous or other criminal actions.

Lawyers typically drew on brain scans and neuropsychological tests to reduce defendants' sentences, but in a substantial number of cases the evidence was used to try to clear defendants of all culpability. "What is novel is the use by criminal defendants to say, essentially, that my brain made me do it," Farahany said following an analysis of more than 1,500 judicial opinions from 2005 to 2012.

The rise of so-called neurolaw cases has caused serious concerns in the country where brain science first appeared in murder cases. The supreme court has begun a review of how such evidence can be used in criminal cases. But legal and scientific experts nevertheless foresee the trend spreading to other countries, including the UK, and Farahany said she was expanding her work abroad.

The survey even found cases where defendants had used neuroscience to argue that their confessions should be struck out because they were not competent to provide them. "When people introduce this evidence for competency, it has actually been relatively successful," Farahany said.

Few cases turn solely on neuroscience evidence, but scans and other techniques have swayed judgments in the past. In 2009, an Italian woman called Stefania Albertani pleaded guilty to murdering her sister, setting fire to the corpse and later attempting to kill her parents. She received a life sentence, but in 2011 Judge Luisa lo Gatto at a court near Milan considered new evidence based on brain scans and genetics. Experts argued that Albertani's crime was driven by abnormalities in the anterior cingulate gyrus, which is involved in impulsivity, and the insula, which has been linked to aggression. The judge reduced Albertani's sentence to 20 years.

Despite the fact that the science is often poorly understood, and that some experts say it is too flimsy to use in court, such evidence has succeeded in reducing defendants' sentences and in some cases clearing them of guilt altogether.

The number of neurolaw cases rose from 100 to 250 a year over the eight-year survey. In 2005, neuroscience appeared in 30 felony cases that did not involve homicide. That number rose to more than 100 in 2012.

Evidence submitted to the US courts ranged from accounts of head injuries to apparent structural or functional abnormalities picked up by brain scans. Lawyers argued that these affected defendants' behaviour by making them more violent, more impulsive, or incapable of planning a crime.Some defendants escaped death sentences on the basis of neurological evidence. Others complained of poor legal assistance when their lawyers failed to have them tested for brain impairments.

Farahany said judges and lawyers urgently needed educating in neuroscience to understand its uses and limitations: lie tests based on brain scans are not infallible, and many brain studies on "criminal minds" draw statistical conclusions from populations and cannot reliably be applied to individuals.

"Law asks questions that science can't answer, and science answers questions that law doesn't ask. You can't leap from a dynamic brain scan to notions of responsibility," said Nigel Eastman, professor of law and ethics in psychiatry at St George's, University of London.

"If you look at functional brain imaging of psychopaths, there's emerging evidence that as a population, people measured as psychopaths psychologically show some slightly abnormal brain scans, but that doesn't mean you can take an individual and do a brain scan and say, 'He's got an abnormal brain.'"

But there are cases where abnormalities in the brain cause criminal behaviour. In 2002, Russell Swerdlow and Jeffrey Burns, neurologists at the University of Virginia medical centre, reported the case of a 40-year-old schoolteacher from Virginia who developed sudden, impulsive paedophilia and was convicted of child molestation. He was signed up for rehabilitation, but was kicked off for propositioning staff.

The evening before the man was sentenced, he complained of a headache and being unsteady on his feet. He was taken to hospital, where doctors found an egg-size tumour in his right orbitofrontal cortex. Once surgeons had removed the tumour, the man's urges disappeared and he was allowed home.

When the man later started collecting child pornography, an MRI scan found that his tumour had grown back. His behaviour returned to normal when the tumour was removed for the second time.

One problem facing the legal system centres on the definition of responsibility. In a rising number of cases, defendants have argued that even though they committed a crime, they cannot be held responsible because their brains made them impulsive, or violent, or incapable of premeditating a crime.

"The question is, how do we best use this evidence in ways that are appropriate, while recognising there are areas where we do things wrong in criminal law and need to improve upon them?" said Farahany.

"A lot of early failures of this evidence in criminal cases could lead to a bias against its validity and its use for a long time to come, so using it for inappropriate claims and stretching the science beyond what it actually says can be devastating," she added.