At birth, infants begin to show basic emotional behaviors, including interest. Interest expressions can include wide eyes, focused attention to something, and eyebrow movements or furrowing. Most babies show interest expressions more often than any other expression over the first 9 months of life.
Izard, C. E., Fantauzzo, C. A., Castle, J. M., Haynes, O. M., Rayias, M. F. & Putnam, P. H. (1995). The ontogeny and significance of infants’ facial expressions in the first 9 months of life. Developmental Psychology, 31(6), 997