The body map

What each system expects, and what it gets

Forty-three reference topics carry a mismatch card: one body system, what it evolved to expect, what the modern environment supplies instead, and what the gap does. Twenty-three of those systems have a place on a body, and they are marked below. The rest do not, and are listed on their own.

43 mismatch cards23 marked on the figureHow the grades work

23 of the book’s mismatch cards have a place on the body. Select a marker to read one. Every card is printed in full below.

Brain · Topic 3

Probable

The Expensive Organ

The brain and the mental habits it runs.

What it expects: Cooked, varied food; sleep; a band of about 30 people inside a wider network; daily physical and mental challenge.
What it gets: Long hours seated (Chapter 6), isolation for some, and prices, deadlines and risks its defaults never met.
What happens in the gap: Defaults misfire as worry, the planning fallacy and reluctance to part with what you own; isolation and inactivity are associated with smaller brain structures and higher dementia risk.

Topic 3: the brief, the evidence and the sources

The 23 systems on the figure, head to foot

Brain · Topic 3
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.
The Expensive Organ
The brain and the mental habits it runs.
What it expects
Cooked, varied food; sleep; a band of about 30 people inside a wider network; daily physical and mental challenge.
What it gets
Long hours seated (Chapter 6), isolation for some, and prices, deadlines and risks its defaults never met.
What happens in the gap
Defaults misfire as worry, the planning fallacy and reluctance to part with what you own; isolation and inactivity are associated with smaller brain structures and higher dementia risk.
Topic 3: the brief, the evidence and the sources
Reward loop · Topic 11
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.
The Hijacked Reward System
The reward system (dopamine-driven wanting, opioid and endocannabinoid liking) and the appetite it steers.
What it expects
Sugar that costs effort or comes wrapped in fruit, ethanol at a fraction of a percent, and food that takes time to chew.
What it gets
Ultra-processed food supplying 55% of American calories, and alcohol in concentrated drinks.
What happens in the gap
Faster eating and about 500 kcal a day of extra intake in a controlled trial; 2.6 million alcohol-attributable deaths a year.
Topic 11: the brief, the evidence and the sources
Body clock · Topic 23
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.
Sleep and the Dark You Have Lost
The circadian clock, set through melanopsin-carrying retinal cells, and the sleep it schedules.
What it expects
A bright day, a dark night, a falling evening temperature, and the same schedule tomorrow.
What it gets
Days under 100 lux indoors, evenings at 30 to 300 lux, a lit bedroom, a schedule that moves at weekends.
What happens in the gap
Sleep moves later and grows irregular, the melatonin night shortens, and peripheral clocks fall out of step.
Topic 23: the brief, the evidence and the sources
Eye · Topic 24
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.
The Indoor Eye
Emmetropization, the visual feedback loop that grows the eyeball to match its optics.
What it expects
A childhood of bright outdoor light, thousands of lux for hours a day, with things to look at near and far.
What it gets
Years of classrooms at a few hundred lux, and 87% of time spent inside a building.
What happens in the gap
The eye overgrows, distance blurs for good, and a stretched retina carries lifelong risk.
Topic 24: the brief, the evidence and the sources
Ear · Topic 26
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.
The Ear in the Silent Room
Hearing, and the subcortical threat circuit the auditory nerve feeds.
What it expects
Intermittent loud events against a quiet, information-rich natural background, plus music made in groups.
What it gets
Continuous mechanical noise, including at night, and music delivered privately through earbuds.
What happens in the gap
Low-grade stress activation with cardiovascular consequences, avoidable hearing loss on a population scale, and a social technology reduced to a solitary one.
Topic 26: the brief, the evidence and the sources
Airway · Topic 10
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.
The Breath You Forgot How to Take
Breathing: the nose as the default airway, and the breath as a lever on heart rhythm.
What it expects
An open nose at rest and in sleep, with the mouth joining in for hard effort.
What it gets
Noses often blocked by allergy, which pushes breathing to the mouth, and breathing hacks that outpace their trials.
What happens in the gap
When the mouth takes over in sleep, the throat collapses more easily: snoring, apneas and oxygen dips.
Topic 10: the brief, the evidence and the sources
Mouth · Topic 15
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.
The Tongue and the Nose
Taste and smell, the chemical senses that judge food and air.
What it expects
Sugar wrapped in fiber, bitter and sour plants, and a nose used daily to find food and judge freshness.
What it gets
Sugar dissolved in drinks at all hours, debittered crops, sweeteners, polluted air, smoke, viruses and an underused nose.
What happens in the gap
Easy sugar overconsumption, lost bitter warnings, and unnoticed smell loss linked to higher mortality.
Topic 15: the brief, the evidence and the sources
Face and jaw · Topic 16
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.
The Shrinking Face
The face, jaws, teeth and upper airway.
What it expects
Hard chewing from childhood, teeth that wear flat, a clear nose, little free sugar.
What it gets
Soft processed food, fork-cut mouthfuls, industrial flour and sugar, and more body weight pressing on the airway.
What happens in the gap
Jaws that fall short of their teeth (crowding, impacted wisdom teeth), retained overbites, widespread decay, and possibly, alongside weight, some sleep apnea.
Topic 16: the brief, the evidence and the sources
Lymph nodes · Topic 9
EmergingEvidence grade: Emerging. Early, small, animal-only, or actively contested.
The Invisible River
The lymphatic system: self-pumping vessels, 500 to 600 nodes, drains around the brain.
What it expects
Kilometers of walking a day, shifting postures, regular nights of sleep.
What it gets
Unbroken sitting, long-haul seats, short nights and, for some, severe obesity.
What happens in the gap
Swollen feet, more clots on long journeys, lymphatic failure at very high BMI; after sleep loss, amyloid rises of uncertain meaning.
Topic 9: the brief, the evidence and the sources
Brown fat · Topic 25
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.
The Thermoneutral Trap
Thermoregulation: brown fat, muscle heat, skin blood flow, and the heat shock response.
What it expects
A day and a year that leave the thermoneutral band: cold mornings, hot afternoons, cooling nights.
What it gets
18°C to 25°C indoors, all year, for about 87% of the time.
What happens in the gap
Cold-recruited tissue and whole-body acclimatization fade within a few years of the signal stopping, as the haenyeo showed, though the metabolic cost is small.
Topic 25: the brief, the evidence and the sources
Immune system · Topic 18
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.
The Immune System's Education
The immune system and the tolerance it must learn.
What it expects
Early years full of microbes from siblings, animals, soil, water and food, and early tastes of lifelong foods.
What it gets
Small families, indoor lives, fewer animals and, until recently, advice to delay allergenic foods.
What happens in the gap
Tolerance fails outward as allergy and inward as autoimmune disease, diagnosed in about one person in 10 in the UK over two decades.
Topic 18: the brief, the evidence and the sources
Heart · Topic 8
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.
The Heart That Remembers the Savanna
The heart and blood vessels, a pump built to move large volumes of blood for hours.
What it expects
Hours of light and moderate activity daily, little salt, plenty of potassium, a lean body and low LDL for life.
What it gets
Long sitting, about 11 g of salt a day on the global average, too little potassium, abundant food.
What happens in the gap
Blood pressure that climbs with age, earlier and commoner atherosclerosis, a lower aerobic ceiling; 1.4 billion adults aged 30 to 79 with hypertension.
Topic 8: the brief, the evidence and the sources
Liver · Topic 14
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.
The Liver and the Chemical Century
The liver, first stop for most of what the gut absorbs: chemical defense and fuel storage.
What it expects
Daily plant defense chemicals, occasional natural toxins such as aflatoxin, and food energy that took work to get.
What it gets
Surplus calories from sugary drinks and alcohol, and more than 350,000 registered chemicals and mixtures, a few of them, such as the long-lived PFAS, beyond its power to break down.
What happens in the gap
About 30% of adults worldwide have fatty livers, and more of them die of heart disease than of liver disease; the main PFAS linger for years in nearly every American's blood.
Topic 14: the brief, the evidence and the sources
Fuel store · Topic 13
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.
The Thrifty Body
Energy storage and appetite: fat, the hormones that report on it, and the muscle, liver and pancreas that handle fuel.
What it expects
Food that takes work to get, lean seasons, fiber-rich plants and hours of physical work.
What it gets
Cheap, energy-dense, ultra-processed food all day, little physical work and short sleep.
What happens in the gap
Fat accumulates, a body built to defend its stores resists losing it, and fat that overflows into liver, muscle and pancreas is thought to drive insulin resistance and type 2 diabetes.
Topic 13: the brief, the evidence and the sources
Stress axis · Topic 28
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.
Stress and the Hijacked Alarm
The stress response: the fast sympathetic arm and the slower hypothalamic-pituitary-adrenal axis.
What it expects
Threats that arrive, get answered with the body, and end, among allies.
What it gets
Uncontrollable social evaluation without end, in steep hierarchies, carried into the night on short sleep and thin company.
What happens in the gap
Rhythms flatten, tissues stop hearing cortisol's stand-down order, inflammation runs on, wounds close slowly and primed arteries fail sooner.
Topic 28: the brief, the evidence and the sources
Kidneys · Topic 17
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.
The Water You Forgot to Drink
Water balance: thirst, kidneys and vasopressin.
What it expects
Wet food and drinking to thirst, in an ape that turns over 30% to 50% less water than its cousins, size for size.
What it gets
Taps, climate control, a 1945 allowance stripped of its caveats, and a market selling hydration.
What happens in the gap
Little, for most healthy people. In long events, overdrinking can dilute blood sodium and swell the brain; thirst lags in older people and in heat.
Topic 17: the brief, the evidence and the sources
Gut · Topic 12
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.
The Gut That Expects Dirt
The gut and its roughly 38 trillion bacteria, which ferment what your own enzymes cannot digest.
What it expects
Tens of grams of fiber a day from many plants, a seasonal menu, and microbes from soil and food.
What it gets
About 17 g of fiber a day (the US adult average), a narrow and refined diet, courses of antibiotics, and sanitized surroundings.
What happens in the gap
Fewer species and less fermentation, with links to disease still being untangled.
Topic 12: the brief, the evidence and the sources
Skin · Topic 19
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.
The Largest Organ
Skin: pigment shield, vitamin D factory, barrier and habitat.
What it expects
Sun matched to its pigment, changing gradually with the seasons, mostly outdoors and often in shade.
What it gets
Pale skin under subtropical sun, dark skin at high latitudes, indoor lives and sudden, burning doses.
What happens in the gap
Skin cancer where sun outruns pigment; low vitamin D, and at the extreme rickets, where pigment outruns sun.
Topic 19: the brief, the evidence and the sources
Reproduction · Topic 32
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.
The Reproductive Paradox
The reproductive axis, from the hypothalamic pulse generator to ovaries and testes, and the post-reproductive life course built around it.
What it expects
Steady energy availability, first births while fertility is at its peak, kin nearby to subsidize weaned children, and a lifetime of relatively few ovulatory cycles (Chapter 22).
What it gets
Energy surplus or deficit rather than balance, a mean first birth at 27.5 in the United States, grandparents in other cities, and an industrial chemical environment nobody has fully characterized.
What happens in the gap
Ovulation stops in women who look healthy, obesity disturbs the same loops from the opposite side, sperm counts and testosterone drift downward across cohorts, and the biological window opens and closes on a schedule the social one ignores.
Topic 32: the brief, the evidence and the sources
Pain · Topic 21
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.
The Pain You Were Not Supposed to Keep
Pain, the body's protective alarm, and the opioid and endocannabinoid systems that quiet it.
What it expects
Injury, then a return to use while still sore, among people who help.
What it gets
Rest, scans that find "damage" in pain-free people, fear and, for a time, freely prescribed opioids.
What happens in the gap
A sensitized alarm keeps ringing after tissue heals, and ordinary pain hardens into disability.
Topic 21: the brief, the evidence and the sources
Leg muscle · Topic 6
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.
The Sitting Disease
Leg muscles, bones and joints, and the blood-sugar control that working muscle provides.
What it expects
Hours of walking and carrying, and rest in postures (squatting, kneeling, ground-sitting) that keep leg muscles working.
What it gets
About 7.7 sedentary hours a day for the average American, and too little activity for nearly a third of adults worldwide.
What happens in the gap
Higher blood sugar after meals within hours; lost fitness within weeks; bone loss within months when loading stops, and lighter skeletons over millennia; higher mortality where long sitting meets low activity.
Topic 6: the brief, the evidence and the sources
Knee, spine · Topic 2
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.
Standing Up
The upright, running, sweating body: spine, knees, springy tendons and sweat glands.
What it expects
Kilometers of walking most days, some running, carrying and throwing, and warm air that sweat can cool.
What it gets
Chairs, cars and desks; nearly a third of adults worldwide below minimum activity.
What happens in the gap
Built-in weak points meet modern ways of loading, moving and weighing: low back pain is the world's leading cause of years lived with disability, and knee osteoarthritis in the United States has doubled beyond what age and weight explain.
Topic 2: the brief, the evidence and the sources
Foot · Topic 7
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.
The Foot That Forgot the Ground
The foot, a sensing, spring-loaded base for walking, running and balance.
What it expects
Bare or thinly shod feet on varied ground, working the arch and toes daily.
What it gets
Cushioned, arch-supported, toe-sprung shoes on flat floors from early childhood.
What happens in the gap
Less ground feel, softened impacts of unknown cost, likely weaker foot muscles and, in some studies, flatter arches; in old age, weak toes and poor balance are linked to falls.
Topic 7: the brief, the evidence and the sources

Systems without a place on the body

A figure can hold an organ, a tissue or a sense. It cannot hold a life course, a village, a landscape or a habit. These 20 topics carry the same mismatch card as the ones above, and are left off the drawing rather than pinned to a limb that would misdescribe them.

Topic 1
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.
The 200-Million-Year Warm-Up
The mammalian operating system: warm blood, night-built senses, milk, late-added color vision.
What it expects
High energy costs paid for by effortful foraging; real darkness at night; changing temperatures; food judged by color and smell.
What it gets
Abundant calories, climate-controlled rooms, bright evenings, engineered food colors.
What happens in the gap
Sleep and light (Chapter 23), temperature (Chapter 25), food reward (Chapter 11); even the body's thermostat is drifting, for unknown reasons.
Topic 1: the brief, the evidence and the sources
Topic 4
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.
The Last Ice Age and the Birth of Agriculture
The agricultural body: a forager's physiology, partly retuned since farming began.
What it expects
A varied diet of wild plants and animals, daily walking and carrying, small mobile groups, seasonal variety.
What it gets
Cereals supplying 42% of the world's dietary energy, sedentary work, and dense settlements shared with animals and strangers.
What happens in the gap
In early farmers, on average: shorter stature, more tooth decay and infection, heavy repetitive labor. Since then, immune genes have been retuned against new pathogens, possibly at the cost of more inflammatory and autoimmune risk.
Topic 4: the brief, the evidence and the sources
Topic 20
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.
The Fever and the Old Enemies
Fever, sickness behavior and the defenses against infection.
What it expects
Infection as the commonest threat, care from others while sick, and bands too small for crowd diseases.
What it gets
Crowd and herd diseases, fever treated as the enemy, work while sick, antibiotics overused and vaccines refused.
What happens in the gap
Fever phobia and contagious presenteeism; returning measles; resistant bacteria; old genetic trade-offs meeting new viruses.
Topic 20: the brief, the evidence and the sources
Topic 22
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.
Cancer in a New Environment
Cell division and the defenses that keep cells cooperating (tumor suppressors, DNA repair, cell suicide).
What it expects
A body of ancestral size and lifespan; menarche in the mid-teens, many pregnancies, years of breastfeeding; no tobacco and little alcohol.
What it gets
Longer lives; earlier menarche, fewer and later births and shorter breastfeeding (three to four times the Dogon's menstrual cycles); excess body fat; tobacco and alcohol.
What happens in the gap
More hormone-driven, obesity-related, tobacco and alcohol cancers; much of the remaining rise reflects age and closer looking.
Topic 22: the brief, the evidence and the sources
Topic 27
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.
The Green Prescription
The stress axis, directed attention and the cardiovascular system, tuned by a living landscape.
What it expects
Daylight hours among plants, water and animals, with shade and people nearby.
What it gets
Most of life spent indoors, a view of a wall, and a city hotter and louder than the land it replaced.
What happens in the gap
Slower stress recovery, tired attention, more psychiatric diagnoses among children raised in the least green places, and extra heart and lung deaths where the trees go.
Topic 27: the brief, the evidence and the sources
Topic 29
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.
The Social Animal in Solitary Confinement
The social brain and the stress physiology it regulates, including the threat circuitry that registers exclusion.
What it expects
Daily, face-to-face, reciprocal contact with a small stable band, inside a wider network of roughly 150 known people.
What it gets
Fewer confidants than a generation ago, most waking contact mediated by screens, and a follower count standing in for a circle.
What happens in the gap
Chronic activation of threat and stress systems, and higher mortality in isolated and lonely people across dozens of cohorts.
Topic 29: the brief, the evidence and the sources
Topic 30
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.
The Attention Economy and the Boredom Deficit
The reward circuit that learns from unpredictable payouts, and the default mode network that runs during unstructured time.
What it expects
Rewards that are earned, spaced and occasionally absent, separated by long intervals with nothing to attend to.
What it gets
A pocket device engineered for time on device, paying out on a variable schedule and filling every gap in the day.
What happens in the gap
Checking behavior that resists extinction, and the loss of the idle minutes in which the brain's inward work has always been done.
Topic 30: the brief, the evidence and the sources
Topic 31
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.
The Mind That Cannot Find Its Village
The defensive emotions, low mood, anxiety and vigilance, plus the attention systems that tune exploring against exploiting.
What it expects
Threats that are physical, local and resolvable; problems a small group can help solve; a world where scanning and moving pay.
What it gets
Unresolvable threats delivered continuously, thin social support, and settings that reward long stillness and narrow focus.
What happens in the gap
Alarms built to over-fire go off constantly and stop resetting, and trait variation that once paid in one environment is diagnosed as pathology in another.
Topic 31: the brief, the evidence and the sources
Topic 33
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.
Born Too Soon
Human birth, and the first months of an infant who arrives unfinished.
What it expects
A labor attended continuously, several pairs of hands afterwards, a founding microbial community from the birth canal, milk carrying sugars for the infant's bacteria, and sleep within reach of an adult.
What it gets
More than one birth in five by cesarean, staff who rotate, formula without the same sugars, a mother recovering largely alone, and in many homes an infant asleep behind a closed door.
What happens in the gap
A different founding microbiome, missing feeding chemistry, more surgical births than medical need explains in some regions and too few in others, and caregiving loads that no ancestral mother carried by herself.
Topic 33: the brief, the evidence and the sources
Topic 34
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.
The Children's Mismatch
The developing skeleton, brain and fear response during childhood's calibration windows.
What it expects
Most of the daylight hours in unsupervised, mixed-age, child-directed play, outdoors, with graded doses of real risk.
What it gets
Adult-scheduled, age-segregated, refereed activity indoors, with risk engineered out and a screen in the gaps.
What happens in the gap
Bone mass unbanked before the window closes, fear left uncalibrated, and an afternoon that a phone ban empties but does not refill.
Topic 34: the brief, the evidence and the sources
Topic 35
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.
The Hallmarks of Aging
The cellular machinery of aging, named in 2023 as twelve hallmarks, from telomere attrition and disabled autophagy to chronic inflammation and dysbiosis.
What it expects
Muscles loaded most days, food that comes and goes, microbes, and stressors that end.
What it gets
Decades of low mechanical load, continuous food, and stress without resolution.
What happens in the gap
Strength falls away faster than age alone requires, while the processes that age cells run with fewer of the brakes the body evolved to apply.
Topic 35: the brief, the evidence and the sources
Topic 36
StrongEvidence grade: Strong. Multiple randomized controlled trials, or consistent large cohort studies with a well-understood mechanism.
Why We Die the Way We Do
All of them, counted at the end: what people die of, and how long they are unwell first.
What it expects
A body that moves, eats, defends, senses and socializes in the conditions of Parts Two to Seven, with death arriving mostly from outside (infection, accident, violence) after a modal age in the seventies.
What it gets
Those conditions removed one at a time, with medical treatment supplied for each resulting failure.
What happens in the gap
Ten causes take 70.9% of US deaths, six of them substantially modifiable, and the United States carries the world's widest healthspan-lifespan gap at 12.4 years.
Topic 36: the brief, the evidence and the sources
Topic 37
StrongEvidence grade: Strong. Multiple randomized controlled trials, or consistent large cohort studies with a well-understood mechanism.
The Individual Variable
Local genetic adaptation: the regional tuning of digestion, metabolism, blood and breath laid down over the last few thousand years.
What it expects
The diet, altitude, sunlight and pathogens of the place where your recent ancestors lived and reproduced.
What it gets
Global food supply, air travel, latitude changes within a single lifetime, and a consumer test that reports ancestry as if it reported physiology.
What happens in the gap
Mostly small mismatches (the dairy that disagrees, the flush after one glass), occasionally serious ones (sickle trait far from malaria, vitamin D deficiency far from the equator), and a steady traffic in genetic advice that outruns the evidence.
Topic 37: the brief, the evidence and the sources
Topic 39
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.
The Unified Field of Mismatch
The whole set at once: immune signaling, the autonomic nervous system, metabolism, sleep and mood, linked through systemic chronic inflammation and the vagus nerve.
What it expects
Inflammation as an emergency, switched on for an infection or a wound and switched off after.
What it gets
A standing low-grade version of the same signal, fed by inactivity, poor diet, disrupted sleep, chronic stress, pollutants and infection.
What happens in the gap
Systems that evolved to trade with each other trade downward together, so a single-target fix often works against the rest of the wheel.
Topic 39: the brief, the evidence and the sources
Topic 40
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.
The Handrail I: Move, Eat, Drink
Skeletal muscle, heart and lungs, the gut and the thirst reflex, all of them signal-driven.
What it expects
Several hours a day of unremarkable movement, occasional hard bursts, loads carried, food that spoils, and water when thirsty.
What it gets
About 5,100 steps, chairs, engines, food engineered to be eaten quickly, and a hydration industry.
What happens in the gap
Systems tuned by use drift downward in their sleep, and the drift shows up decades later as disease.
Topic 40: the brief, the evidence and the sources
Topic 41
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.
The Handrail II: Light, Sleep, Temperature, Nature
The circadian clock, thermoregulation, and the stress-recovery loop, all driven by environmental signals rather than conscious effort.
What it expects
Thousands of lux by day, near-total darkness at night, a temperature that falls after sunset, occasional cold and heat, and daily hours outdoors.
What it gets
Under 1,000 lux of waking light in the week the campers spent at home, evenings in the 30 to 300 lux band the clock reads most steeply, a thermostat that never moves, and most of a life indoors.
What happens in the gap
The clock drifts later and flattens, sleep timing scatters, and systems tuned by temperature and daylight have nothing to tune against.
Topic 41: the brief, the evidence and the sources
Topic 42
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.
The Handrail III: People, Attention, Meaning
The social and reward circuitry that regulates contact, touch, attention and purpose.
What it expects
Daily presence among a small, stable group; touch as a routine event; effort before reward; a role the group depends on.
What it gets
A wide, thin network delivered through glass, touch made optional, reward decoupled from effort, and purpose left to arrange itself.
What happens in the gap
Inner-layer relationships go unfed without anyone noticing, stress physiology loses a regulator, and low purpose and low participation track higher mortality in large cohorts.
Topic 42: the brief, the evidence and the sources
Topic 43
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.
The Mismatch Within the Mismatch
Decision-making and habit formation, built to run on context and imitation rather than daily conscious analysis.
What it expects
An environment that supplies the right behavior by default, so that eating, moving, sleeping and resting require no deliberation.
What it gets
An environment that supplies none of them, plus a $6.8 trillion market selling deliberation back as the remedy.
What happens in the gap
Health becomes a permanent cognitive task; vigilance curdles into orthorexia and orthosomnia in some people, and into abandonment in many more.
Topic 43: the brief, the evidence and the sources
Topic 44
EmergingEvidence grade: Emerging. Early, small, animal-only, or actively contested.
The Luxury Trap Accelerates
Effortful cognition: memory retrieval, navigation, reasoning and the cognitive reserve that a lifetime of such effort appears to build.
What it expects
Daily problems that must be solved with the equipment inside the skull, in a world where nobody else holds the answer.
What it gets
An external store for every fact, a route for every journey and, lately, a competent draft of almost any piece of thinking, available before the effort starts.
What happens in the gap
Unknown at the level of brain health, and measurable already at the level of learning: work that looks better while the person doing it learns less.
Topic 44: the brief, the evidence and the sources
Topic 45
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.
Designing the Bridge
Human plasticity, the body's lifelong capacity to re-tune itself to whatever signals its surroundings supply.
What it expects
A place whose ordinary defaults deliver load, daylight, darkness, heat, cold, microbes and company without anyone having to decide on them.
What it gets
Environments built around cars, screens, shelf life and margin, with the missing signals sold back as effort, subscription or supplement.
What happens in the gap
Plasticity keeps working, in the direction the environment points, and health becomes a permanent act of resistance against the street you live on.
Topic 45: the brief, the evidence and the sources

Where to go next

Evidence grades on this page:StrongEvidence grade: Strong. Multiple randomized controlled trials, or consistent large cohort studies with a well-understood mechanism.ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.EmergingEvidence grade: Emerging. Early, small, animal-only, or actively contested.