fMRI data can evidence a strong relationship between the structure of the brain and a psychological function or “mental state”, a key notion in the field of penal law. However fMRI is a method that can show correlations, but not causality links between a brain region and cognitive or psychological functions. Judges often make the mistake of interpreting neuro imagery data as objective evidence of mental illness, by assuming that the diagnosis of the illness can be made on the basis of the brain images. However, although neuro imagery can today assist in diagnosis of neurological and psychiatric illnesses, no study has been able to show a causal relationship between certain anomalies in brain structure or function and a particular mental illness