"Negative reactivity" refers to the tendency of reacting or responding in a negative or unfavorable way. It means that someone tends to have a pessimistic or critical reaction when faced with certain situations or stimuli. Full definition
The development of negative reactivity in irritable newborns as a function of attachment. (cambridge.org)
Regarding ODD and CU in preschoolers, Willoughby et al. [32] reported that CU traits were stable from ages 3 to 5, and distinguished a group of children with ODD+CU that were less fearful, recovered more easily after an upset, and showed less negative reactivity, lower heart period reactivity and higher levels of general arousal than those with ODD only. (journals.plos.org)
Behavior problems are, in general, characterized by negative reactivity [100]. (file.scirp.org)