Toddlers' speech and cognitive effort. 
Two-year-olds were videotaped while engaged in free play and problem-solving play.  The periods in which they were actively manipulating objects were analyzed as to level of engagement, distinguishing between periods of intense engagement, in which the children were concentrating on the nonverbal activity, and periods of casual engagement, in which they conducted the nonverbal activity in a more leisurely fashion.  A micro-analysis of the children's spontaneous speech revealed that they spoke less at moments of intense engagement than at moments of casual engagement.  These and other findings are taken as support for the hypothesis that when toddlers invest cognitive effort in nonverbal behavior, there is a reduction in mental resources available for speech.