Wikipedia names two different years for two different events (1910, the start of Assagioli's thinking; 1927, the first publication of the named system) and does not say which one the field treats as the founding date.
What would resolve this A history-of-psychotherapy source that states explicitly which of the two years, or a third year, the field itself treats as psychosynthesis's founding.
Openhistory of psychotherapyWikipedia: Psychosynthesis