Morphological systems are constrained in how they interact with each other. One case that has been widely studied in the psycholinguistic literature is the avoidance of plurals inside compounds (e.g. *"rats eater" vs. "rat eater") in English and other languages, the so-called "plurals-in-compounds effect." Several previous studies have shown that both adult and child speakers are sensitive to this contrast, but the question of whether semantic, morphological, or surface-form constraints are responsible for the plurals-in-compounds effect remains controversial. The present study provides new empirical evidence from adult and child English to resolve this controversy. Graded linguistic judgments were obtained from 96 children (age range: 7;06 to 12;08) and 32 adults. In the task, participants were asked to rate compounds containing different kinds of singular or plural modifiers. The results indicated that both children and adults disliked regular plurals inside compounds, whereas irregular plurals were rated as marginal and singulars as fully acceptable. Furthermore, acceptability ratings were found not to be affected by the phonological surface form of a compound-internal modifier. We conclude that semantic and morphological (rather than surface-form) constraints are responsible for the plurals-in-compounds effect, in both children and adults.