The lack of scientific attention for HRM within SMEs can be seen as a scale effect in HRM research. In addition, two more scale effects can be identified: scale effects in the actual management of human resources and scale effects in the impact of HRM practices. This paper discusses these three scale effects, in order to answer the following research questions:

− Does the return on HRM investments depend on firm size?

− Why do small firms pay less attention to HRM than larger firms do?

− Why has HRM research tended to ignore small and medium-sized enterprises?