Gravitationally unstable, shocked layers of interstellar gas are produced by cloud-cloud collisions and by expanding nebulae around massive stars. We show that the resulting fragments are likely to be of high mass (≳7 Msun), and initially well separated (i.e. weakly bound to one another, if at all).This result may explain why dynamically active regions tend to have a high efficiency of massive star formation, and why they tend to relax quickly into a self-propagating mode which generates sequences of OB subgroups. These tendencies are manifested on many scales, from local star-forming regions like Orion, through regions like 30 Doradus in the LMC, to the most IR-luminous starburst galaxies. We also show that, for a wide range of input parameters, gravitational fragmentation of a shocked layer occurs when the column density of hydrogen nuclei through the accumulating layer reaches a value ∼6 × 1021 cm-2. This may be one reason for the mass-radius relation for molecular cloud clumps first noted by Larson.