A man accused of killing a 15-year-old boy he thought was a witch had brain damage, a court had been told.
Neuropsychiatrist Dr Peter Fenwick told the Old Bailey a scan of Eric Bikubi's brain showed lesions which "probably contributed to an abnormal mental state at the time of the offence".
Mr Bikubi and his partner Magalie Bamu, both 28 from Newham, east London, deny murdering her brother Kristy Bamu.
He drowned on Christmas Day 2010 during an exorcism, the trial has heard.
Mr Bikubi has admitted manslaughter on the grounds of diminished responsibility, which the prosecution has rejected.
Kristy and his siblings were visiting the couple for Christmas, but Mr Bikubi had accused the boy and two of his siblings of witchcraft, the court heard.
All three were beaten and other children were forced to join in the attacks, the jury has been told.
But it was Kristy who became the focus of Mr Bikubi's attention, the prosecution said.
Jurors were told Kristy died when he was "too exhausted" to resist and keep his head above water when Mr Bikubi put him in a bath. Mr Bikubu realised too late Kristy was not moving.
Mr Bikubi has admitted assaulting Kristy's siblings.
Ms Bamu denies the murder charge, as well as two charges of causing actual bodily harm to her other siblings.
The trial continues.