Cover legend: Receptor autoradiograms illustrating the distribution of oxytocin receptors in the brain of monogamous prairie voles. Natural variation in oxytocin receptor density in the nucleus accumbens (left two columns) is correlated with variation in affiliative behavior. Increasing oxytocin receptor expression in the nucleus accumbens using viral vector gene transfer (right column) accelerates the formation of partner preferences in females. For more information, see the article by Ross et al. in this issue (pages 1312–1318).