Simplify the following text for a Grade 4 native English-speaking person.
Original: Stevenson 's research was the subject of Tom Shroder 's Old Souls : The Scientific Evidence for Past Lives ( 1999 ) and Jim B. Tucker 's Life Before Life : A Scientific Investigation of Children 's Memories of Previous Lives ( 2005 ) .
Simplified: Stevenson 's research was the subject of Jim B. Tucker 's Life Before Life ( 2005 ) .