At birth, infants can respond to human voices. They are also capable of distinguishing between the sounds of different languages, and they begin to show a preference for the language spoken by their mothers soon after birth. In one experiment, infants of English-speaking mothers and infants of Spanish-speaking mothers were given a pacifier that could record how often they sucked. Then the researchers linked the infants' sucking patterns to recordings of women's voices, so that when the infants increased or decreased their rate of sucking they heard a woman speaking in English or a woman speaking in Spanish. Both groups of infants controlled their sucking to hear more recordings of a woman speaking in the same language as their mothers, showing that they preferred this language to other languages.