More than 40 million Americans suffer from some form of mental illness, and nearly 10 million struggle with a serious condition such as chronic depression, bipolar disorder, and schizophrenia.

While most people with mental illness are high-functioning in their daily lives, more serious conditions can take an enormous toll on health, quality of life, and economic productivity. Worse, many people who are mentally ill do not receive the care they need. According to an estimate by the National Alliance on Mental Illness, over half of those afflicted with bipolar disorder or schizophrenia go untreated.

4. Ohio

> Pct. of adults w/ serious mental illness: 5.1%
> Total adults w/ serious mental illness: 459,000 (5th most)
> Pct. adults abusing alcohol or illicit drugs: 9.3% (18th highest)
> Median household income: $51,075 (17th lowest)

Nearly 460,000 Ohioans suffer from a serious form of mental illness, including schizophrenia, bipolar disorder, bulimia, anorexia, and major depressive disorders. About 7.6% of the state’s adult population has had a major depressive episode in the past year alone.

Ohio, like many states, is battling a heroin and prescription painkiller epidemic. Drug abuse is correlated with mental health issues because it can lead to the development of disorders and aggravate existing conditions and because those who have disorders often self-medicate to alleviate symptoms. In March, Ohio Gov. John Kasich proposed that opioid prescriptions in his state be limited to seven days, potentially combatting abuse.

Many types of mental illness are hereditary, but genetics is not the only contributing factor. Many other factors can lead to the development of mental health disorders, including substance abuse, traumatic experiences, the lack of personal connections, and stress. Combined, these factors can lead to mental illness being more common in certain states.

In several of the states on this list, a high share of adults reports alcohol or illicit drug abuse or dependence, and many have very high rates of drug overdoses. In New Hampshire, which has the highest share of adults with a serious mental illness, about one in 10 residents 18 years and older report substance abuse — the second highest share in the country. In West Virginia, which has the second highest rate of serious mental illness, the drug overdose rate is the highest in the country, and double the national rate.

Those who exhibit symptoms of mental illness are much more likely to report substance abuse. People experiencing mental illness are more likely to self-medicate as a way to alleviate symptoms. Substance abuse can also lead to the development of mental illness, or the exacerbation of existing symptoms.

Stress can also lead to the development of severe depression and other psychological disorders. Individuals in poverty are more likely to face stressful conditions. For example, they are more likely to live in high crime areas and to struggle with financial difficulties — conditions that over time have been shown to lead to the development of serious mental illness.

Of the 14 states with the highest shares of adults with a serious mental illness, 11 have median household incomes below the national figure of $55,775 a year.

Americans with certain health conditions, such as diabetes and heart disease, are more likely to develop a psychological disorder. Of the 14 states with high rates of serious mental illness, nine have above average diabetes incidence rates, and 12 have higher heart disease rates than the national average.

While states with the highest incidence of serious mental illness are alike in some respects, mental illness can affect people of any economic background, gender, race, or social and health status.

Click here to see the 14 states struggling with mental illness.

Click here to read our methodology.