A well-known effect in speech production is that more predictable words tend to be phonetically reduced. Recent work has suggested that predictability effects result from hardwired properties of the language production system, rather than active modulation by the talker to accommodate the listener. However, these studies investigated only minor manipulations of listener characteristics. Here, we examine predictability effects with two very different listener populations: adults and preverbal infants. Using mixed effects regressions on spontaneous speech corpora, we compare the effect of word frequency, probability in context, and previous mention on word duration in adult-directed and infant-directed speech. We find that the effects of preceding context and word frequency differ according to listener. Contrary to previous work, these results suggest that talkers do modulate the phonetic effects of predictability based on listener characteristics. To our knowledge, this study is also the first published analysis of predictability effects in infant-directed speech.