This paper challenges the historiographical discontinuity established between earlier “anthropometric testing” and the arrival of “psychological testing” with Binet and Simon's intelligence test in 1905. After some conceptual clarifications, it deals with “word association”: a kind of psychological experimentation and testing which became popular over the last two decades of the 19th century. First Galton's exploration are presented, followed by experiments performed at the Leipzig laboratory by Trautscholdt, and then Cattell and Bryant's collective testing. Additionally, I document the use of this method for the study of mental difference through the works of Münsterberg, Bourdon, Jastrow, Nevers and Calkins. The cases I present show how the method gave rise to various measurements and classifications. I conclude that the word association technique triggered reflection on mental “uniqueness”, gender traits and the influence of education, among other topics. Moreover, it prepared the terrain and anticipated some basic attractions and problems intelligence testing would later encounter.