Psychology › Development & attachment › Lifespan development
Dementia and Alzheimer's disease
Described 1906; first disease-slowing drugs 2023· Condition or disorder
Well replicatedStrong, repeated evidence.
A decline in memory and thinking, caused by brain disease, severe enough to disrupt independent daily life.
See it on the Psychology map →What it says
Dementia is an umbrella term; Alzheimer's disease causes roughly 60–70% of cases, and vascular, Lewy body and frontotemporal dementia most of the rest. In 1906 the German psychiatrist Alois Alzheimer described plaques and tangles in the brain of a patient, Auguste Deter. About 57 million people worldwide live with dementia (WHO, 2021). There is no cure, but lecanemab and donanemab, approved in 2023 and 2024, modestly slow early Alzheimer's, and good care and support help.
Why it matters
It is a leading cause of disability in old age. Early evaluation matters: some memory problems have treatable causes, and legal and financial planning is easier while judgment is intact.
The evidence
Status: well replicated. Autopsies since 1906, brain scans and spinal-fluid and blood tests that detect amyloid and tau, and trials such as lecanemab's 2022 study of about 1,800 patients, which slowed decline 27%.
Open questions
Whether amyloid is the root cause: the new drugs clear plaques yet slow decline only modestly, so tau, inflammation and blood vessels likely matter too.
Inside this (1)
Connections (7)
▸Hippocampus: forming new memories is damaged early in Dementia and Alzheimer's diseaseWell replicated
The connection
In Alzheimer's disease, tau tangles appear early in and around the hippocampus, the region that turns new experiences into lasting memories. That is why the first sign is often forgetting recent conversations or events while old memories stay intact. The damage then spreads through wider cortex, affecting language, reasoning and eventually daily tasks.
Who showed it and when
The spread pattern was mapped by Heiko and Eva Braak in Frankfurt in 1991 from autopsied brains; MRI studies since the 1990s show hippocampal shrinkage in early disease.
The evidence
Autopsy staging, MRI volume measurements that track memory decline, and the parallel with patient H.M., who could not form new memories after hippocampal surgery.
Open questions
Why the hippocampal region is so vulnerable, and why some people with heavy plaque and tangle loads stay sharp, perhaps thanks to cognitive reserve.
▸Sleep loss, the body clock and health is linked to higher risk of Dementia and Alzheimer's diseaseMixed evidence
The connection
During deep sleep the brain appears to clear waste, including amyloid, the protein that builds plaques in Alzheimer's. Even one night without sleep raises amyloid levels in healthy adults, and long-term studies link short sleep in midlife to more dementia later. But poor sleep is also an early symptom of dementia, so cause and effect may run both ways.
Who showed it and when
Maiken Nedergaard's University of Rochester team reported in 2013 that the brain's 'glymphatic' cleaning system works mainly during sleep in mice; a 2021 Whitehall II analysis followed nearly 8,000 British civil servants.
How solid is it?
A 2018 NIH scan study found one lost night raised brain amyloid about 5%. Whitehall II linked persistently sleeping six hours or less, at ages 50, 60 and 70, with about 30% higher dementia risk.
Open questions
Whether better sleep actually lowers dementia risk, which no trial has shown yet, and whether glymphatic clearing works in humans as claimed; a 2024 mouse study challenged it.
▸Dementia and Alzheimer's disease erode Memory and how it worksWell replicated
The connection
Dementia usually strikes memory first, in a typical order: recent events and new information go before old memories, which can stay vivid for years. Learned skills and habits, stored in other brain systems, often last longest, which is why someone may still play piano after forgetting the day's visit. Later, language, judgment and recognizing people decline.
Who showed it and when
Observed since Alzheimer's 1906 case of Auguste Deter, who said 'I have lost myself'; mapped in detail by memory clinics and research centers worldwide since the 1980s.
The evidence
Standard memory tests track the decline year by year, and brain scans tie it to damage in the hippocampus and later the wider cortex.
Open questions
Why musical and emotional memories often survive so long, and why some dementias, such as frontotemporal, spare memory early and change personality first.
▸Dementia and Alzheimer's disease are not a normal part of Aging: what declines and what doesn'tWell replicated
The connection
Age is the biggest risk factor, but dementia is disease, not normal aging. Normal aging means slower recall, like misplacing keys or reaching for names. Dementia means losing the ability to manage daily life: getting lost on familiar routes, struggling with bills, repeating questions. About a third of Americans over 85 have Alzheimer's; most do not.
Who showed it and when
The distinction hardened after neurologist Robert Katzman's 1976 editorial argued that much 'senility' was Alzheimer's disease, reshaping research and funding.
The evidence
Long-term studies show many people reach their 90s and 100s mentally intact, and autopsies tie dementia to specific diseases rather than age itself.
Open questions
Whether everyone would eventually develop dementia if they lived long enough; rates keep rising into the late 90s, and the answer is unknown.
▸Modifiable dementia risk factors could delay or prevent many cases of Dementia and Alzheimer's diseaseMixed evidence
The connection
The Lancet Commission estimated that eliminating 14 modifiable risk factors could, in theory, prevent or delay about 45% of dementia cases. Even delaying onset by a few years would sharply cut the number of people living with dementia. Falling age-specific dementia rates in the US and Europe suggest change is possible.
Who showed it and when
Gill Livingston of University College London led the Lancet Commission reports of 2017, 2020 and 2024; the US Framingham study has tracked falling rates since the 1970s.
How solid is it?
The 45% is a modeling estimate that assumes causation; trials such as FINGER and SPRINT MIND show smaller but real benefits from lifestyle and blood-pressure programs.
Open questions
How much is achievable in practice; risk factors cluster with poverty, and no trial has yet prevented dementia itself at large scale.
▸Loneliness and social isolation raises the risk of Dementia and Alzheimer's diseaseMixed evidence
The connection
Lonely, isolated older adults develop dementia at higher rates. Possible routes include less mental stimulation, more stress hormones and inflammation, less exercise, and depression. A 2024 Lancet commission listed social isolation among 14 modifiable risk factors for dementia.
Who showed it and when
Large cohort studies in the US, UK and elsewhere since the 2000s; the Lancet Commission on dementia prevention (2017, 2020, 2024).
How solid is it?
A 2024 meta-analysis of over 600,000 people linked loneliness to about a 30 percent higher dementia risk, but these are associations, not experiments.
Open questions
Early dementia itself can cause withdrawal, so part of the link may run backward; whether reducing loneliness lowers risk has not been proven.
▸Fraud and scams: why they work especially target people with Dementia and Alzheimer's diseaseWell replicated
The connection
Dementia erodes judgment, memory and the ability to spot deception, often before diagnosis, while trust and politeness remain. Scammers share 'sucker lists' of people who have responded before, so victims with cognitive decline get targeted again and again. Missed bills and unusual money transfers are often early warning signs.
Who showed it and when
Studies of US older adults: the Rush Memory and Aging Project in Chicago (2019) and a 2021 analysis of Medicare and credit records.
The evidence
Rush researchers found low scam awareness predicted later Alzheimer's diagnosis; credit records show missed payments rising up to six years before diagnosis.
Open questions
How to protect people without stripping their independence. Brokerages can now pause suspicious withdrawals and call a 'trusted contact,' but uptake is uneven.