From the perspective of attachment theory the importance of studying daycare is self-evident: infants and young children are separated from their parents, to be cared for by professional caregivers in a noisy group setting, if only for a few hours per day and for a few days per week. Sarah Hrdy called infant group care provided by non-biologically related professional caregivers an evolutionary unique experiment which should be carefully investigated. Human infants are not evolutionary adapted to professional group care, and although many children might be responding well to the potential strains and stresses of daily group care, some children might be less robust. We found that children with a reactive temperament may be doing great in high quality daycare but showed most behavior problems in low quality care. The theory of differential susceptibility suggests markers of susceptibility to negative environments that might lead to better fit between child characteristics and type of daycare.