In 2009 in "Mothers and Others", Hrdy explored cognitive and emotional implications for infants growing up in what was (for an ape) a novel developmental context. Instead of relying on the single-minded dedication of their mothers, youngsters had to monitor and engage multiple caretakers as well. Other apes possess cognitive wiring for rudimentary Theory of Mind, but with cooperative rearing, relevant potentials for mentalizing would have become more fully expressed, and thus rendered more visible to natural selection. Over generations, those youngsters better at inter-subjective engagement would have been best cared for and fed, leading to directional Darwinian selection favoring peculiarly human capacities for intersubjective engagement.