In 1887, he wrote a book entitled "Mental Affections of Childhood and Youth". It was published at the request of the Medical Society of London and was a transcript of three lectures along with fifteen papers Down published on mental defects. The book details his ideas and findings about several mental abnormalities such as Down syndrome and microcephaly. It also contains his view on the leading thoughts and available literature on the subject. In the lectures and some of the papers, he also weighed in on what he believed were the potential causes of various mental disorders. A recurring theme was the influence of parental physical and mental health on their child's chances of being born with a mental disorder. He also explored how the obstetric practices of the time could have influenced postnatal health.