A psychologist says Christie Marceau's killer could have disguised his insanity when being psychiatrically assessed.
Akshay Chand has been deemed insane and therefore not guilty on a charge of murdering the Auckland teenager.
But he's been sentenced to three years for kidnapping her, two months before he killed her while released on bail after being ruled sane.
Auckland University professor of psychology Glynn Owens says a person appearing to be sane and then not two months later isn't unusual.
"A person can hear the voices in their head and just, basically keep secrets about them. And if they don't tell you about them, you're not going to know and you may well think - most of the time this person seems quite sane and quite sensible."
Glynn Owens says people with varying degrees of psychiatric illness can manage to cover this up over a large amount of the time.
And he says the sentence Christie Marceau's killer has received could keep him off the streets longer than if he were in prison.
Mr Owens says although there's been a not guilty verdict, Chand is going to have a much tougher time there than in prison.
"Being detained indefinitely, could mean he spends the rest of his life locked up, which is very unlikely if you are convicted on criminal charges."
Photo: Christie Marceau (Supplied)