1. The operation of ‘negative density-dependence' in seedling cohorts in tropical forests is empirically well-established, but only at a phenomenological level that leaves open the question of why seedlings conspecific with an overtopping parent tree experience higher mortality than heterospecifics. The distinction is of theoretical importance because distinct mechanisms are involved. 2. We consider the two most debated possibilities: seedling mortality resulting from classical Lotka-Volterra density-dependence and seedling mortality resulting from the action of biotic agents, as postulated for the Janzen-Connell mechanism. Our study is the first to identify the full spectrum of mortality factors affecting both conspecific and heterospecific members of seedling cohorts. 3. We took advantage of the occurrence of ‘seedling carpets', dense concentrations of seedlings that appear following fruiting events under reproductive individuals of some species. In these ‘carpets', seedlings conspecific with the overtopping parent tree predominate numerically, but heterospecific seedlings are also typically present. Here, we investigated the differential survival of conspecific versus heterospecific seedlings under focal trees of four species: Calatola microcarpa (Icacinaceae), Clarisia racemosa (Moraceae), Matisia cordata (Bombacaceae/Malvaceae) and Sorocea pileata (Moraceae). 4. We show that mortality rates of conspecific seedlings are much higher than those of heterospecific seedlings and that most conspecific mortality (64-100%) resulted from host-restricted arthropod herbivores and/or fungal pathogens, whereas the mortality of heterospecific seedlings resulted from a variety of other causes. 5. Synthesis. Conspecific seedlings died following attack by apparently host-restricted arthropods or fungi and eventually experienced 100% mortality. The results are inconsistent with classical intra- and inter-specific competition and consistent with the actions of distance-responsive and/or density-responsive ‘enemies', as postulated 40 years ago by Janzen and Connell.