Although animal studies report a reduction in pain intensity under fear, studies on humans yield conflicting results. For example, in some cases decreased pain was observed but not in others.
A likely explanation for this discrepancy could be found in the involvement of anxiety in pain. Notably, research shows that fear and anxiety are mediated through two distinct neural circuits (i.e., brain regions) which underscores the qualitative difference between these two emotions.
Furthermore, previous studies suggest that the actual experience of intense pain induces fear and a reduction of that pain, while in contrast the expectation of pain elicits anxiety or, more specifically, anticipatory anxiety and amplifies the perception of pain.
Supporting this statement, victims of traumatic stress experience numbness and insensitivity to pain during the fear eliciting trauma, while patients with generalized anxiety disorder, who are constantly hyper vigilant about their internal body states, report amplified pain intensity. The current study replicates those results in a healthy population by showing that fear increased the participants’ pain threshold , while anticipatory anxiety decreased it. Therefore, the authors conclude that fear induces analgesia, while anxiety produces hyperalgesia.
Rhudy JL, Meagher MW. Fear and anxiety: divergent effects on human pain thresholds. Pain. 2000 Jan;84(1):65-75. doi: 10.1016/s0304-3959(99)00183-9. PMID: 10601674.