The sound of animal vocalizations is shaped by communicative function and evolutionary history. In this talk I will describe research that examines two kinds of vocal signals in humans: infant-directed (ID) speech and spontaneous laughter. Acoustic features of ID speech are functionally specialized for getting infants’ attention and communicating intentions. While cultural variation exists in the frequency of its occurrence, the production and recognition of ID speech is similar across quite different societies. Analysis of the adaptive functions of ID speech provides the best level of explanation for its acoustic structure. In the case of laughter, mechanistic and phylogenetic factors can play a more central role in understanding its structural features, particularly in the acoustic differences between spontaneous laughter and volitional laughter which provide differential cues of speaker intent in social interaction. I will conclude with some thoughts about how Tinbergen’s four questions are differentially useful depending on the specific behavior one is examining.