Table 1.

Hidden variables in neuropsychopharamcology research, their impact on study outcomes, and suggested solutions.

Problems	Impact	Solutions
Light/dark phase Circadian effects	Physiological effects [8–10, 13, 18, 21–23] Drugs interaction effects [13–17, 20] Learning tasks [17, 25, 26] Pain perception [24]	Testing behavior during dark phase [10] Keeping all types of lighting dim during testing, including red light [11, 12]
Transportation Stress	Immune function [29] Reproduction [40, 41] Development [33–35]	Breed animals in lab or avoid shipping during puberty Consider “two-hit” model if shipping pregnant animals [45] Allow time for animals to acclimate before testing [31, 32]
Home Cage Environment	Cold Stress [51, 53] Hormone levels [52, 55–57], Behavior [60–64]	If forced air caging is unavoidable, add extra bedding or enrichment for thermoregulation [66, 68] Avoid behavioral testing on cage change days [60, 61] Intermix experimental groups that are housed on separate levels [63]
Social Environment	Hormone levels [76, 77] Aggressive behaviors [73–75, 78] Evolutionary history [71, 72, 91–93]	Record social ranking within cage [64, 82, 87, 88, 90] House animals based on species specific behavior Use alternative strategies to house animals after surgery recovery