A guest post defending the amyloid hypothesis of Alzheimer's disease, explaining why amyloid is likely the root cause despite recent criticism and discussing why early treatments have only been partially effective. Longer summary

David Schneider-Joseph presents a detailed defense of the amyloid hypothesis of Alzheimer's disease, which states that the disease is caused by accumulation of amyloid-β peptide. He explains the ATN model (amyloid → tau → neurodegeneration) and provides extensive evidence from genetics, clinical studies, and animal models. The post addresses various criticisms of the hypothesis, including the recent research fraud controversy and apparent failures of early treatments. The author explains why current treatments have only achieved about 30% efficacy and predicts that future treatments targeting amyloid earlier and more effectively could achieve much better results. Shorter summary

Scott Alexander defends psychiatry's use of 'chemical imbalance' theory, arguing it was meant to emphasize depression's biological basis rather than claiming a simple serotonin deficiency. Longer summary

Scott Alexander responds to criticisms of psychiatry's use of the 'chemical imbalance' theory of depression. He argues that psychiatrists never claimed depression was simply a serotonin deficiency, but rather a complex interaction of brain chemicals. He explains that the term 'chemical imbalance' was used to emphasize depression's biological basis and help people take it seriously, rather than viewing it as a personal failure. Scott provides evidence that serotonin and other neurotransmitters are indeed involved in depression, while acknowledging the full picture is more complicated. He concludes that framing depression in terms of brain chemistry remains useful, even if 'chemical imbalance' is no longer the best terminology. Shorter summary