PME, the International Group for the Psychology of Mathematics Education, was founded in 1976, at the "Third International Congress on Mathematical Education" in Karlsruhe, organised by the International Commission on Mathematics Instruction (ICMI). While PME is thus beyond coming of age and is reflecting its further orientation--due to the present "social turn"--the origins of investigating psychological aspects of mathematics learning have not yet been systematically studied. I am undertaking here a first such approach, concentrating on Germany, where the first pertinent monographs were published in 1913 and 1916. Different endeavours, focussing in particular on the notion of error, merged into the characteristic approach of "experimental pedagogy". Given the key function of ICMI for founding PME, an additional aspect is whether the forerunner of ICMI: the "Internationale Mathematische Unterrichtskommission" (IMUK), founded in 1908, had an impact upon promoting research into the psychology of mathematics education. The pertinent research was effected by psychology; doing research themselves was still outside the horizon of mathematics educators. Perspectives of future research, in particular comparative ones, are outlined.