Adverse Childhood Experiences (ACEs) is the term used to describe traumatic experiences before age 18 that can lead to negative, lifelong emotional and physical outcomes.

When the stress of these adverse experiences is so severe or prolonged that a child is unable to process it, what should be a normal survival response becomes “toxic stress”. This type of stress alters the functioning of the brain and has a long-lasting and injurious impact on the developing mind, which we call “trauma”. This trauma affects the way those suffering it think and act throughout their lives. Understanding such mental and emotional trauma is key to understanding the behaviour of millions of people.

The term ACEs derives from a study carried out in the 1990s in California. The 10 ACEs they measured were:


Subsequent ACE studies have added other traumatic experiences to this list. 

What the researchers were testing was whether experiences in childhood can impact various different health issues in later life, and to what extent. What they found was not only extraordinary, but has been confirmed by numerous studies of hundreds, thousands and tens of thousands ever since.

Prevalence of ACEs

The original study found the following prevalence for their list of 10 ACEs:

ACE Category


Percent (N = 17,337)

Emotional Abuse


Physical Abuse


Sexual Abuse


Mother Treated Violently


Household Substance Abuse


Household Mental Illness


Parental Separation or Divorce


Incarcerated Household Member


Emotional Neglect*


Physical Neglect*


*Collected during Wave 2 only (N=8,629).


Using the results, the researchers decided to create an ACE score. Exposure to any single ACE counted as one point, with each ACE measured equally. Points were then totalled for a final ACE score. For instance, a person who was a victim of physical abuse, witnessed domestic violence and whose parents separated would have an ACE of 3. It is important to note that the ACE score does not capture the frequency or severity of any given ACE in a person’s life, focusing instead on the number of ACEs experienced.

Here is a breakdown of ACE scores for four studies which have been carried out in the UK since 2013:

What outcomes do ACEs affect?

As the number of ACEs a person has experienced increases, so does the risk for the following:

  • Alcoholism and alcohol abuse

  • Chronic obstructive pulmonary disease

  • Depression

  • Foetal death

  • Health-related quality of life

  • Illicit drug use

  • Ischaemic heart disease

  • Liver disease

  • Poor work performance

  • Financial stress

  • Risk for intimate partner violence

  • Multiple sexual partners

  • Sexually transmitted diseases

  • Smoking

  • Suicide attempts

  • Unintended pregnancies

  • Early initiation of smoking

  • Early initiation of sexual activity

  • Adolescent pregnancy

  • Risk for sexual violence

  • Poor academic achievement

This list, taken from the website of the US Centre for Disease Control and Prevention (who co-ran the initial ACE study), is far from exhaustive. Countless studies have shown that ACEs can affect a seemingly endless range of factors spanning physical health, mental health, lifestyle choices and behaviour. When all is taken into account, it could be said they have the potential to affect virtually every aspect of a person’s life.

ACEs are also of concern because they can:

  • Increase the body’s ‘allostatic load’ – wear and tear through fluctuating or heightened neural or neuroendocrine responses.
  • Have a direct impact on a child’s health e.g. through inflicted physical injury.
  • Normalise such behaviours as violence, assault and abuse.
  • Result in behaviours offering short-term relief at the expense of longer-term health (e.g. smoking, harmful alcohol consumption, poor diets and early sexual activity).
  • Increase risk of transmission to the next generation – the intergenerational cycle.


How strong is the relationship between ACEs and these outcomes?

ACEs don't only increase the risk of developing health-harming behaviours, mental issues and diseases, but do so to an extraordinary extent.

 Compared to those with no ACEs, those with 4+ ACEs are:

2 times more likely (currently) to binge drink and have a poor diet

3 times more likely to be a current smoker

5 times more likely to have had sex before they were 16

6 times more likely to have had, or caused, an unplanned teenage pregnancy

7 times more likely to have been involved in violence in the previous year

11 times more likely to have used heroin/crack, or been incarcerated

6 ACEs increased …

by 46 times: the risk of becoming an IV drug user, and

by 35 times: the risk of suicide


Though the exact figures change in different studies, the tendency for likelihoods to rise by hundreds of percentage points as the ACE score creeps up is uniform among them. Which outcomes hold the strongest associations with ACEs were laid out in a 2017 meta-analyses of ACE studies across the world, funded by the Welsh Government, which found that:

  • Strongest: problematic drug use, and interpersonal and self-directed violence.
  • Strong: sexual risk-taking, mental ill health and problematic alcohol use.
  • Moderate: smoking, heavy alcohol use, poor self-rated health, cancer, heart disease and respiratory disease.
  • Weak or modest: physical inactivity, overweight or obesity and diabetes.


What became clear in this analysis was that the outcomes most strongly associated with multiple [4 or more] ACEs also represent ACE risks for the next generation (e.g. violence, mental illness, and substance use).

It’s also worth noting that what constitutes a weak or modest correlation in ACEs studies is usually an impressive leap in others. For instance, physical inactivity was found to be the weakest of all the connections analysed in this meta-analyses, yet if you look at the results of the initial study’s first wave, your chances of having no leisure-time physical activity jump by 144% between 0 to 4+ ACEs (from 18.4% to 26.6%). That a near 50% increase could be considered a weak correlation is testament to how much of an impact ACEs can have on many other outcomes.

Why ACEs have the effect they do

What is most important about an adverse childhood experience is whether each incident creates “toxic stress”. “Toxic stress” is where a person experiences natural stress responses (fight, flight or freeze), but in a manner which is more prolonged, severe or unpredictable than usual. For instance, it is the difference between boxing in a gym, but returning home to a stable household; and being punched by your father, creating an unsafe home environment.

When this is coupled with a lack of support by caring adults and a deficit of other protective resilience factors [hyperlink], the brain is left unprotected from the increased levels of cortisol and development is disrupted, thereby increasing the risk for stress-related disease and cognitive impairment well into the life course. The impact of this is most detrimental during the first years of life as this is the period in which the brain has the highest capacity for change.

All of this comes down to the brain’s well-intentioned desire to survive in the moment and how the decisions made at this point continue long past the initial experience, leaving the person years later stressed about threats that no longer exist and fighting old dangers years after they’ve passed.

Take John for example. John is a seven-year-old whose dad beats his mum in front of him every other night. His brain reacts to the threat in the moment by pumping adrenaline and cortisol into his head. But because this is persistent, and because there’s no adult there to protect him, his brain ‘learns’ that he is in danger at all times. And so it adapts to this constant threat, pumping these chemicals into John’s head more regularly and at higher levels during moments of perceived threat. This then becomes the brain’s normal way of functioning.

Over time, this constant stress causes John to develop anxiety. He later becomes a heavy smoker and drinker, his mind drawn to both substances as coping mechanisms for this over-reactive survival response. He becomes an alcoholic and spends his days distracted, depressed and unproductive at work. John’s anxiety leads to him developing a meek personality, making him easy prey for violent partners, keeping him trapped in the same situation his mother once was. Over the years, all this stress, smoking, drinking, depression and violence take their toll on John’s health, all contributing towards the heart disease he develops. He dies prematurely, capping off a lifetime of sad circumstances whose roots began in his childhood.