As a result the man will not be eligible for psychiatric care and may be sentenced to prison, newspaper Falu Kuriren reports.
While admitting to the murder, the man also claims that he experienced certain memory lapses.
The dead couple, a woman aged 62 and a man aged 59, were found in their house by police after another of the couple’s sons said he had not been able to contact them for several days.
The 18-year old was previously unknown to the police and has no previous convictions.
He remains in custody while awaiting formal charges.