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Two people can live through the same difficult year and end up in very different places. One develops insomnia, then panic attacks, then a depression that drags on for months. The other bends and recovers. Differences in risk, whether biological, social or environmental, explain a good part of that gap.
About one in eight people worldwide lives with a mental disorder, and roughly half of all lifetime cases show up by age 14. That timing matters. Most of the factors below start shaping the brain long before anyone books a first appointment with a therapist.
1. Childhood adversity and early trauma
In the mid-1990s, researchers at Kaiser Permanente asked more than 17,000 adults about ten categories of childhood hardship: physical abuse, emotional neglect, a parent with mental illness, a home where no adult could be relied on for food or safety. Each one counted as a point. People with four or more points had far higher rates of depression, suicide attempts and addiction, and a life expectancy years shorter than those who scored zero.
Early adversity doesn’t only leave difficult memories behind. It recalibrates the stress system. A child raised in an unpredictable home often develops a nervous system that stays on alert, which later looks like anxiety, poor emotional regulation and a low tolerance for ordinary pressure.
Why the timing matters
The first few years of life are the most plastic. Consistent, responsive caregiving builds the circuitry that calms a person down after something frightening. When that care is missing or chaotic, the same circuitry learns a different lesson: threat is constant, so stay ready. That lesson can be revised later, but it takes deliberate work.
2. Stress that never lets up
Acute stress is useful. It sharpens attention, gets you through an exam or a crisis, then fades. Chronic stress is a different animal: caring for a parent with dementia, working a job where you have no say over your schedule, carrying debt for years, living in a relationship where you’re braced for the next argument.
Sustained cortisol exposure is linked to reduced hippocampal volume and measurably worse working memory. If you’ve noticed your recall slipping during a rough stretch, this breakdown of the causes of memory loss and what each one does to the brain is useful context, because several of those causes are temporary.
Occupational research is blunt here. Jobs that combine high demand with low control, such as call centre work, some hospital wards and long-haul driving, show higher rates of depression and anxiety than jobs with similar hours but more autonomy. It’s the lack of control more than the workload that corrodes wellbeing.
3. Isolation and loneliness
Loneliness isn’t the same as being alone. You can feel isolated in a crowded office and perfectly content on a quiet Sunday by yourself. What damages health is the gap between the contact you have and the contact you want.
A 2010 meta-analysis pooling 148 studies and more than 300,000 people found that strong social relationships were associated with a 50% greater likelihood of survival across the follow-up period. That’s comparable to the effect of quitting smoking. Loneliness also hides inside other explanations: people blame their personality or their age when the real problem is that nobody has their number. It’s one of the risk factors for depression most people ignore, and one of the few that can shift within a month.
4. Chronic physical illness
The relationship runs both ways. Pain, fatigue and disability wear down mood. Depression then worsens outcomes for diabetes, heart disease and stroke, partly through inflammation and partly through missed medication and skipped appointments. Several of the most common diseases worldwide carry a documented mental health burden. Around one in three people with diabetes reports significant depressive symptoms, and persistent pain conditions roughly triple the odds of an anxiety disorder.
Cardiovascular disease deserves its own mention. Depression after a heart attack is common and predicts a worse prognosis, which is why cardiac rehab programmes increasingly screen for it. Knowing the coronary heart disease symptoms, treatment and prevention basics helps for the obvious reason, and also because unexplained chest tightness and exhaustion can be anxiety rather than ischemia. The two get confused constantly, in both directions.
5. Alcohol and other drugs
Alcohol is a depressant that people drink to feel less depressed, which is the trap. The nightly glass of wine that smooths an evening fragments sleep by 3am, drops mood the next afternoon, and invites another glass. Heavy drinking raises the risk of depression, and drinking to manage anxiety keeps both problems running.
Cannabis follows a similar pattern for a smaller group. Risk climbs with early, frequent and high-potency use, and it climbs steeply for people who already have a family history of psychosis. For someone with that history, daily use from age 15 is one of the more avoidable risk factors on this list.
6. Family history and genetics
Heritability estimates sit around 80% for schizophrenia, 60 to 80% for bipolar disorder, and 35 to 40% for depression. Those numbers can sound like a verdict. They aren’t. If one identical twin develops schizophrenia, the other’s risk is roughly 48%, which is high but nowhere near certain. Genes set sensitivity; environment decides how much of it gets expressed.
Family history is best used as a screening clue rather than a prediction. A sibling with bipolar disorder or a parent who was hospitalised in their twenties is information a good GP should have on file, especially before prescribing anything.
7. Disrupted sleep
Insomnia is both a symptom and a driver. People with chronic insomnia have roughly double the risk of developing depression later on, and sleep problems often appear weeks before a first depressive episode. Shift work, untreated sleep apnoea and a schedule that swings by four hours between weekdays and weekends all count.
The encouraging part: in trials of older adults with insomnia, those who completed cognitive behavioural therapy for sleep were about half as likely to develop depression over the following year. Fixing sleep is one of the few places where prevention is genuinely practical.
How risk actually accumulates
None of these factors works alone. A person with a difficult childhood, a demanding job and no close friends carries a very different load from someone with one of the three. That’s why two people with the same diagnosis can need completely different treatment.
Protective factors work the same way, in reverse:
- At least one or two people you could call at an awkward hour and who would actually pick up.
- A sleep schedule that stays roughly consistent, including weekends.
- About 150 minutes of moderate movement a week, which is linked to 20 to 30% lower depression risk in large cohort studies.
- Predictable housing and income, which sound like social policy rather than mental health care until you look at the data.
- Treatment that actually happens: therapy, medication or both, taken and reviewed rather than started and abandoned.
What to do with this list
Go through the seven and note which apply to you right now. Three or more active at once is a reasonable signal to get a checkup rather than wait for symptoms to announce themselves. GPs use short screeners, the PHQ-9 for mood and the GAD-7 for anxiety, and a score takes four minutes to produce.
Certain windows concentrate risk. Bereavement in the first year, divorce, job loss, a new diagnosis, moving cities, and the twelve months after a baby arrives are all periods when a little support goes a long way. If you’re inside one of those windows, treat a low mood that lasts more than two weeks as information, not weakness.
And if you’re the one watching someone else, the most useful question isn’t whether they’re okay. It’s more specific than that: you’ve seemed flat for about three weeks, and you’ve stopped answering messages. What’s going on? Specificity tends to get an honest answer.
Risk factors aren’t a verdict. They’re a map of where to look, and where to push back.
