Robert Ladd
This undated handout photo provided by the Texas Department of Criminal Justice shows Robert Ladd, who was executed Thursday, Jan. 29, 2015. Defense attorneys said they found juvenile records suggesting Ladd was mentally disabled. Texas Department of Criminal Justice

A convicted murderer who was mentally disabled was executed Thursday night in Texas, after the U.S. Supreme Court declined to step in. Defense attorneys said Robert Ladd’s death violates the Constitution and the inmate’s life would have been spared in any other state because he had an IQ of 67, according to a statement from the American Civil Liberties Union, which represented Ladd’s appeal.

"Texas aggressively pursued Mr. Ladd's execution, despite the fact that our Constitution categorically prohibits the use of capital punishment against persons with intellectual disability,” ACLU attorney Brian Stull said in a statement Thursday night. “Mr. Ladd, whose IQ was 67, was executed because Texas uses idiosyncratic standards, based on stereotypes rather than science, to determine intellectual disability. His death is yet another example of how capital punishment routinely defies the rule of law and human decency.”

After Texas judges declined an appeal that the 57-year-old inmate was ineligible for the death penalty, ACLU attorneys petitioned the high court Wednesday for a stay of execution and to review the ruling. The attorneys presented evidence of a Texas psychiatrist’s conclusion in 1970 that Ladd had an IQ score of 67 and was “fairly obviously retarded.” The psychiatrist later reaffirmed this initial diagnosis in an affidavit, stating Ladd’s IQ test and “three separate interviews confirmed my diagnosis of mental retardation,” according to a statement from the ACLU. Defense attorneys argued most U.S. states require an IQ of 70 or below as a cutoff for determining whether an individual has a mental disability.

Ladd was sentenced to death for the 1996 slaying of a 38-year-old mentally impaired woman, who was strangled, bludgeoned, bound and burned in her East Texas apartment. At the time of the murder, Ladd was on parole after serving more than a decade a 40-year prison sentence for the 1980 murder of a Dallas woman, according to the Associated Press.

The Supreme Court has previously ruled that the Eighth Amendment prohibits executing intellectually disabled individuals, but left the determination of what classifies mental retardation up to individual state governments. Texas courts’ criteria for intellectual disability have not been endorsed by the scientific or medical community. Texas has killed more prisoners than any other state in the U.S., with more than 500 inmates executed since 1982, according to the Texas Department of Criminal Justice.

"We are eager for a court to address the fact that Texas' unscientific standards can't be reconciled with the Supreme Court's decision in Hall v. Florida, mandating that states must use universal medical diagnostic practices rather than inaccurate and self-invented methods for determining intellectual disability,” Stull said in a statement Thursday night. “However, no future ruling can undo the unconscionable fact that tonight Texas ended the life of an intellectually disabled man who deserved the protection of the Constitution."

Last year, the high court ruled Hall v. Florida ruled in Hall v. Florida that relying on the state’s IQ threshold requirement for deciding whether a person is eligible for the death penalty is unconstitutional. The Supreme Court upheld standards in Atkins v. Virginia, in which the medical community defines intellectual disability according to three conditions: “significantly subaverage intellectual functioning, deficits in adaptive functioning (the inability to learn basic skills and adjust behavior to changing circumstances), and onset of these deficits during the developmental period.” If an individual meets these criteria, the person is ineligible for execution, the high court previously ruled.

The Supreme Court ordered Wednesday the temporary stay of three Oklahoma inmates’ executions, after hearing their arguments last week challenging the state’s lethal injection procedure. The high court will decide whether the procedure breaches the Eighth Amendment’s prohibition of cruel and unusual punishment, Reuters reported. The order came one day after the Supreme Court declined arguments that Georgia inmate Warren Hill was mentally disabled. Hill was executed Tuesday.