Dr Willemsen, a child, family and adult psychologist, is frequently instructed as an expert-witness in Private and Public Law Proceedings, as well as PLO and Child Protection cases as instructed by Local Authorities. He developed expertise in the assessment of children, young people and their families. Areas of assessment include neglect, sexual abuse and domestic violence, including risk assessments in cases of domestic violence and sexual abuse. He is instructed in cases involving false allegations of sexual abuse, parental alienation and emotional abuse. In addition he developed expertise in the assessment of parents and children with learning disabilities or learning difficulties, to include the use of psychometric assessments.
He gave oral evidence on numerous occasions before Magistrates, District and Circuit Judges, as well as High Court Judges. He provided Court Reports for the States of Jersey and European Jurisdictions.