The Psychiatric and Psychosocial Overlap
Every condition in this chapter shares a history of psychosomatic framing — being dismissed as “not real” or “all in the mind” before biological evidence accumulated. The psychosomatic-lag pattern (how long a contested condition took from first clinical description to biological acceptance) is instructive: multiple sclerosis ~100 years, epilepsy ~2,500 years, rheumatoid arthritis was distinguished from “rheumatism” only with the discovery of RF in 1940.
ME/CFS and its neighbors currently occupy the “contested” phase of this trajectory. The hierarchical relationship between psychiatric and physical diagnosis is a structural feature of the nosological landscape: when a condition lacks a biomarker, it defaults to psychiatric classification (somatic symptom disorder in DSM-5, bodily distress disorder in ICD-11). This has two consequences: (a) it creates a structural incentive for patients to emphasize physical over psychological symptoms (to avoid a psychiatric label that invalidates their illness experience), and (b) it means the same patient could receive a psychiatric diagnosis from one clinician and a physical diagnosis from another, purely as a function of the clinician’s nosological philosophy.
Consequence: The contested-diagnosis cluster is united not just by shared symptoms but by shared structural position — all are conditions that lack biomarkers, affect predominantly women, and occupy the contested boundary between psychiatry and medicine. Addressing the nosological problem requires not just better biomarkers but structural reform of a classification system that assigns conditions to psychiatry by default.
The nosological argument that “markers resolve disputes” has a growing precedent within the psychiatric axis that typically absorbs contested physical diagnoses. A single-cell atlas of the adult human hippocampus (~500,000 nuclei) reported that neurogenesis is stalled in major depressive disorder, and argued for reclassifying depression by its molecular features — analogous to the shift that moved cancer classification from anatomical site to cellular/molecular characteristics (Peng et al. 2026). This reframes the depression-ME/CFS boundary: rather than a hard psychiatric-vs-physical divide, it suggests both conditions may occupy overlapping molecular space that is not yet resolved by a validated biomarker.
The relevance to ME/CFS is indirect and currently unsupported by direct data — there are no published studies of adult hippocampal neurogenesis in ME/CFS. The plausible mechanistic bridges are (a) HPA-axis dysfunction, which is documented in both CFS (Papadopoulos and Cleare 2012) and, in animal models, is coupled to adult hippocampal neurogenesis (Snyder et al. 2011); and (b) post-infectious suppression of hippocampal neurogenesis, demonstrated after mild SARS-CoV-2 infection and linked to cognitive symptoms in long COVID (Fernández-Castañeda et al. 2022). The premise that adult human hippocampal neurogenesis persists at a measureable level is itself contested, with conflicting post-mortem evidence (Boldrini et al. 2018) vs. (Sorrells et al. 2018) (Sorrells et al. 2021). (Certainty: 0.64–0.80 for the depression-axis finding across supporting and competing studies; the ME/CFS extrapolation is a speculation, not an established mechanism.) (Severity applicability: unknown — not stratified by ME/CFS severity.)
Consequence: If depression becomes molecularly subclassifiable, the “is it depression or ME/CFS?” boundary could shift from clinical judgment toward biological testing — but only if ME/CFS neurogenesis is studied directly, which it has not been.
Falsifiability: Direct measurement of dentate-gyrus neurogenic markers (DCX, nestin, Ki67) in ME/CFS post-mortem or surgical hippocampal tissue showing density comparable to matched controls — and no correlation between such markers and ME/CFS fatigue severity — would falsify the proposal that a shared neurogenic suppression links ME/CFS to molecular depression; the claim would also weaken if a consensus assay established that adult human neurogenesis is undetectable by any method.
A molecular atlas showing stalled adult hippocampal neurogenesis in major depressive disorder (Peng et al. 2026) raises the possibility — together with the related observation that a post-infectious state can suppress hippocampal neurogenesis (Fernández-Castañeda et al. 2022) — that primary molecular depression and the reactive low mood of ME/CFS may one day be distinguishable at the cellular level Is Molecular Depression Distinct From Reactive Depression in ME/CFS?, and that a behavioural proxy (pattern separation) might make that distinction testable without tissue Pattern Separation as a Behavioural Marker Distinguishing Molecular From Reactive Depression.
The strongest constraint is the evidence vacuum on the ME/CFS side: there are no direct studies of adult hippocampal neurogenesis in ME/CFS, so every ME/CFS implication here is speculative rather than established Molecular Reclassification of Depression and the Depression-ME/CFS Boundary. The field also disagrees whether adult human neurogenesis is measureable at all, so the entire thread is contingent on resolving that controversy.
What the evidence supports is a reframing: depression is not a single serotonin-deficiency state but a heterogeneous molecular condition, and the depression–ME/CFS boundary may be molecular rather than purely clinical. What remains open is whether any neurogenesis-linked marker can actually discriminate the two in practice.
Consequence: These independent threads point toward the same unresolved question — whether the low mood seen in ME/CFS is molecularly distinct from primary depression — which matters because the two would need different management, but answering it requires direct ME/CFS studies that do not yet exist and a reliable way to measure adult neurogenesis, which the field has not yet agreed on.