By practicing against considerably weaker opposition one may, even unintentionally, become used to lower playing standards; and playing "crooked" chess, in which the strategic principles of play assume a minor role and tactics become paramount, is bound to make you stupid.So, there you have it, tactics alone makes you stupid.
Paul Morphy: A Modern Perspective, 19.
31 May 2012
Beware of Weak Opponents
Referring to Paul Morphy's tendency to play games at odds--a knight or rook down from the start in most cases,--Valeri Beim opines: