Simplify the following text for a Grade 11 native English-speaking person.
Original: Tucker , a board-certified child psychiatrist , worked for several years on this research with Ian Stevenson before taking over upon Stevensonâ s retirement in 2002 .
Simplified: Tucker , a child psychiatrist , worked for several years on this research with Ian Stevenson before taking over upon Stevensonâ s retirement in 2002 .