Freezing as a measure of fear: sex differences and implications for preclinical research.

Rodent fear conditioning paradigms are widely used to model core features of posttraumatic stress disorder (PTSD), with freezing behaviour serving as the dominant readout. However, the extent to which freezing captures biologically and clinically relevant variability, particularly with respect to sex differences, remains unclear. We conducted a systematic review and meta-analysis of studies employing fear conditioning and extinction in wild-type rodents to assess sex differences in freezing behaviour. These were not found across studies. However, freezing levels were significantly correlated with shock intensity only in male rodents. We conclude that freezing may represent a constrained and potentially low-resolution behavioural output, which may fail to detect meaningful variation in fear processing. Given that PTSD exhibits pronounced sex differences in humans, the translational validity of commonly used behavioural endpoints may be questionable. Including both sexes and refining behavioural readouts in animal models are critical for improving the translational pipeline of stress research.