Wikipedia's own Narrative therapy article states only that the approach was developed during the 1970s and 1980s by Michael White and David Epston, with no single founding year named; the Michael White biographical article separately dates the Dulwich Centre's founding to 1983 and the field-defining book to 1990, leaving which year properly marks the school's founding unresolved.
What would resolve this A source that explicitly states which single year the discipline treats as narrative therapy's founding, or an explicit editorial convention here for dating an approach that developed gradually over two decades.
Openhistory of psychotherapyWikipedia: Narrative therapy