Another shooting, another son of divorce. From Adam Lanza, who killed 26 children and adults a year ago at Sandy Hook School in Newtown, Conn., to Karl Pierson, who shot a teenage girl and killed himself this past Friday at Arapahoe High in Centennial, Colo., one common and largely unremarked thread tying together most of the school shooters that have struck the nation in the last year is that they came from homes marked by divorce or an absent father. From shootings at MIT (i.e., the Tsarnaev brothers) to the University of Central Florida to the Ronald E. McNair Discovery...