The ACE Study at 25: What We've Learned Since the Most Important Health Study Ever Conducted

In 1995, Dr. Vincent Felitti was running an obesity clinic at Kaiser Permanente in San Diego when he noticed something puzzling. His most successful patients — those losing weight rapidly — were the ones most likely to drop out of the programme. One woman, who had lost over 50 kilograms in a single year, suddenly regained all of it after being propositioned by a colleague. When Felitti interviewed her, she said something that would alter the trajectory of public health research: the weight was not the problem. The weight was the solution.

That observation led Felitti to partner with Dr. Robert Anda at the Centers for Disease Control and Prevention. Together, they designed what would become the Adverse Childhood Experiences Study — the largest investigation ever conducted into the relationship between childhood trauma and adult health. Between 1995 and 1997, they surveyed over 17,000 predominantly middle-class, college-educated Kaiser Permanente members in Southern California. What they found was so significant that, twenty-five years later, it continues to reshape our understanding of human health and disease.

The ACE Study identified ten categories of adverse childhood experiences, divided into three domains. The first domain covered direct abuse: physical abuse, emotional abuse, and sexual abuse. The second addressed neglect: both physical and emotional. The third encompassed household dysfunction: growing up with a family member who was incarcerated, who struggled with substance use, who had a mental illness, or witnessing domestic violence, as well as parental separation or divorce.

Each category counted as one point. A person's ACE score, ranging from zero to ten, represented how many of these adversities they had experienced before age eighteen. The researchers expected that adverse childhood experiences would be relatively rare among this educated, insured, middle-class population. They were wrong. Nearly two-thirds of participants reported at least one ACE. More than one in five reported three or more. And one in sixteen had an ACE score of four or higher.

The most striking finding was the dose-response relationship. As a person's ACE score increased, so did their risk for virtually every major cause of death and disability in the developed world. Compared with someone who had zero ACEs, a person with four or more was approximately twice as likely to develop heart disease, twice as likely to develop cancer, three and a half times more likely to develop chronic obstructive pulmonary disease, and four and a half times more likely to experience depression. A person with six or more ACEs had their life expectancy reduced by nearly twenty years.

The relationship held even after controlling for behavioural risk factors such as smoking, physical inactivity, and obesity. This was the finding that stunned the medical community. It was not simply that childhood adversity led to unhealthy behaviours, which then led to disease. Something deeper was at work. Childhood adversity appeared to get into the body directly, altering biological systems in ways that created disease decades later.

Felitti's original clinical observation — that weight gain served a protective function — has been borne out by decades of subsequent research. The ACE Study found that a person with an ACE score of four or more was 1.4 to 1.6 times more likely to develop severe obesity. But the relationship was not simply about comfort eating. The biological mechanisms run much deeper.

Chronic childhood stress dysregulates cortisol, which in turn promotes visceral fat accumulation regardless of caloric intake. The hormone neuropeptide Y, released during prolonged stress, directly stimulates fat cell growth. Meanwhile, epigenetic changes — alterations in how genes are expressed without changes to the DNA sequence itself — can modify metabolic set points. Research published in journals including Translational Psychiatry and Biological Psychiatry has demonstrated that methylation patterns on genes governing stress response, inflammation, and metabolic function are measurably different in adults with high ACE scores compared to those with low scores. Some of these epigenetic changes have been shown to be transmissible across generations, meaning the biological consequences of adversity may extend beyond the individual who experienced it.

For Felitti, this confirmed what his patients had been telling him all along. The weight was not a failure of willpower. It was a biological and psychological adaptation to an environment that felt fundamentally unsafe. The body was doing exactly what it was designed to do under conditions of chronic threat: storing energy, maintaining vigilance, and creating a physical barrier between the self and the world.

In the quarter-century since the original study, the ACE framework has been replicated in over forty countries across six continents. The World Health Organization conducted its own multi-country study confirming the dose-response relationship. State-level surveys across the United States, coordinated through the Behavioral Risk Factor Surveillance System, have expanded the data set from thousands to hundreds of thousands of respondents. The findings have been remarkably consistent regardless of geography, culture, or economic context.

The science has also expanded beyond the original ten categories. Researchers have proposed additional ACEs including community-level adversity such as exposure to neighbourhood violence, systemic racism, poverty, and bullying. Dr. Nadine Burke Harris, who served as California's first Surgeon General, became one of the most prominent advocates for integrating ACE screening into routine paediatric care. Her 2018 book and her subsequent policy work helped push California to become the first state to reimburse clinicians for ACE screening through Medi-Cal.

The ACE framework is not without its critics. Some researchers have pointed out that the original study population was predominantly white, middle-class, and insured — not representative of the broader population. Others have argued that the ten categories are too narrow, failing to capture the effects of poverty, racism, and community violence. The binary scoring system, in which each category counts equally and is either present or absent, does not distinguish between a single incident and years of sustained abuse.

There is also concern about the potential for ACE scores to be used deterministically. A high ACE score does not guarantee poor health outcomes. Resilience — shaped by supportive relationships, community resources, and individual neurobiology — plays a significant protective role. Some researchers worry that widespread screening could lead to labelling or stigmatisation without adequate therapeutic resources to follow up.

These are legitimate criticisms. But they do not diminish the central finding: that what happens to us in childhood has measurable, dose-dependent effects on our biology and health across the entire lifespan.

Perhaps the most frustrating aspect of the ACE legacy is how slowly its findings have been translated into practice. Despite twenty-five years of overwhelming evidence, childhood trauma screening remains the exception rather than the rule in most healthcare systems worldwide. Medical training still devotes relatively little attention to the biological embedding of adversity. Insurance and reimbursement structures still favour treating downstream diseases rather than addressing upstream causes.

Felitti himself expressed frustration with this gap. The medical system, he observed, was designed to treat the consequences of adverse childhood experiences — heart disease, diabetes, depression, addiction — without ever asking about the experiences themselves. It was, in his analogy, like finding a house on fire and focusing all resources on the smoke damage rather than locating the source of the flames.

What the ACE Study ultimately revealed is that much of what we call adult disease is not, in fact, a disease of adulthood. It is a consequence of childhood adversity that has been biologically embedded over decades. The body keeps the score, as psychiatrist Bessel van der Kolk famously wrote. The question facing medicine and public health today is whether we are willing to listen to what the body has been trying to tell us for twenty-five years — and to act on what we hear.