Phonological patterns in languages often involve groups of sounds rather than individual sounds, which may be explained if phonology operates on the abstract features shared by those groups (Troubetzkoy, 193957. Troubetzkoy , N. 1939/1969 . Principles of phonology , Berkeley : University of California Press . View all references/1969; Chomsky & Halle, 19688. Chomsky , N. and Halle , M. 1968 . The sound pattern of English , New York : Harper and Row . View all references). Such abstract features may be present in the developing grammar either because they are part of a Universal Grammar included in the genetic endowment of humans (e.g., Hale, Kissock and Reiss, 200618. Hale , M. , Kissock , M. and Reiss , C. 2006 . Microvariation, variation, and the features of universal grammar . Lingua , 32 : 402 – 420 . View all references), or plausibly because infants induce features from their linguistic experience (e.g., Mielke, 200438. Mielke , J. 2004 . The emergence of distinctive features , Ohio State University : Unpublished doctoral dissertation . View all references). A first experiment tested 7-month-old infants' learning of an artificial grammar pattern involving either a set of sounds defined by a phonological feature, or a set of sounds that cannot be described with a single feature—an “arbitrary” set. Infants were able to induce the constraint and generalize it to a novel sound only for the set that shared the phonological feature. A second study showed that infants' inability to learn the arbitrary grouping was not due to their inability to encode a constraint on some of the sounds involved.