We studied ten-fold stacked layers of self-assembled InAs/GaAs quantum dots by photoluminescence in pulsed magnetic fields. When the interlayer distance is reduced from 9.8 to 5.5 nm, a doubling of the diamagnetic shift for a magnetic field perpendicular to the [0 0 1] direction reveals the onset of electron coupling between the dots in the stack. On reducing the interlayer distance to 3.1 nm, a lower exciton effective mass is seen in addition to the coupling. For such a close stacking, the strain field in and around the dot is completely different from that of a single-layered structure. In particular, the strain inside the InAs dots in the stack is partially relaxed, causing the observed effect. (C) 2004 Elsevier B.V. All rights reserved.