The U.S. Supreme Court. (Karen Bleier/Agence France-Presse via Getty Images)

Bob Taft, a Republican, was governor of Ohio from 1999 to 2007. Joseph E. Kernan, a Democrat, was governor of Indiana from 2003 to 2005.

Legislators in six states — Indiana, Ohio, South Dakota, Tennessee, Texas and Virginia — have proposed legislation to prohibit the death penalty for individuals with severe mental illness. As former governors of states that are grappling with this issue, we strongly support this effort to end an inhumane practice that fails to respect common standards of decency and comport with recommendations of mental-health experts.

The overwhelming majority of people with severe mental illness are not violent; in fact, they are more likely to be victims than perpetrators of violent crime. For the very small number who do commit a capital crime while suffering from a severe mental disorder, current death-penalty law does not adequately take the effects of their illness into account.

As a result, defendants with severe mental illness — such as schizophrenia, bipolar disorder, post-traumatic stress disorder and traumatic brain injury — continue to be sentenced to death and executed. Last March, Texas executed Adam Ward, a man recognized as “diagnosed with bipolar disorder and placed on lithium as early as age four,” according to appellate court documents. And in 2015, Georgia executed Andrew Brannan, a decorated Vietnam War veteran who also had a pronounced mental illness. He qualified for 100 percent disability from the Department of Veterans Affairs because of his PTSD and bipolar disorder.

Although their grave illnesses do not excuse these defendants’ crimes, we believe that life imprisonment without the possibility of parole would have been a more appropriate punishment. Illnesses such as schizophrenia and bipolar disorder are characterized by impairments that — when untreated — significantly affect one’s ability to distinguish fact from reality, to make rational decisions or to react appropriately to events and other people. Under these conditions, the degree of culpability may not rise to the level of cold, unimpaired calculus that justifies the ultimate penalty.

In addition, people with severe mental illness are more vulnerable to being wrongfully accused and convicted. Research has shown that such people are overrepresented among cases of false confession because the conditions of their illness — such as proneness to confusion and a lack of assertiveness — may render them more vulnerable to interrogation techniques and impair their ability to invoke their constitutional rights.

Studies have also shown that death- penalty jurors often misunderstand mental illness, which is often viewed as an aggravating factor — that is, a reason to sentence someone to death — rather than as a mitigating factor, which is what it should be. The troubling consequence is that some defendants may end up on death row because of their mental illness.

The fact that the death penalty applies to those with mental illness also means that veterans with demonstrated PTSD may be executed. Even though most of the thousands of veterans struggling with PTSD do not commit the serious crimes that may be eligible for the death penalty, an estimated 10 percent of the United States’ death-row inmates are veterans — some of whom suffered from active and severe symptoms of PTSD at the time of their crime. These veterans have experienced trauma that few others have faced and have made a vital contribution to the safety of our country that deserves our recognition.

Legislation being considered on this topic varies by state, but each bill creates a case-by-case decision-making process — conducted by either a judge or jury — to determine if a defendant has a severe mental illness. Only those with the most serious diagnoses would qualify. The legislation has been endorsed by major state and national mental-health organizations, including the American Psychological Association and the American Psychiatric Association, as well as by faith groups, elected officials and former public safety officials.

The death penalty was not intended for people in the throes of severe delusions, living with schizophrenia or suffering from combat-related PTSD. These are not the blameworthy individuals whose executions can be justified.

We come from different political parties, but we join the majority of Americans — supporters and opponents of the death penalty alike — who believe it should not be imposed on defendants with such serious impairments. This is a fair, efficient and bipartisan reform that would put an end to a practice that is not consistent with current knowledge about mental illness and fundamental principles of human decency.