Since as early as the 1960s, defense lawyers have introduced the idea that people accused of murdering lesbian, gay, bisexual and transgender people had acted in a state of temporary insanity caused and justified by their victim’s sexual orientation or gender identity.
The legal strategy, known as the “gay panic” or “transgender panic” defenses, was not always effective, and as attitudes toward L.G.B.T. people shifted, it was used less often. But it has still been deployed in recent years by lawyers hoping to win a jury’s sympathy, lessen a defendant’s charges or shorten a sentence.
On Wednesday, New York became the seventh State Legislature to approve a ban on such defenses.
The measure’s passage came amid a growing national push to bar the defenses, which gay and transgender rights activists say codify discriminatory attitudes into the legal system. Lawmakers in three other states approved similar bans this year.
Assemblyman Daniel J. O’Donnell and State Senator Brad Hoylman, both gay Democrats from Manhattan, had introduced bills last weekend that would stop people charged with murder from mounting gay panic or transgender panic defenses.
The legislation, which was approved during Pride Month and as New York prepared to commemorate the 50th anniversary of the Stonewall uprising, was the culmination of an effort that started when Mr. Hoylman introduced a similar bill in 2014.
“I’m glad that New York is sending a message to prosecutors, to defense attorneys, juries and judges that a victim’s L.G.B.T.Q. identity can’t be weaponized,” Mr. Hoylman said in an interview.
Gov. Andrew M. Cuomo, who has said he would sign the bill, called the measure “an important win for L.G.B.T.Q. people everywhere.”
Several New York-based criminal defense organizations that opposed the legislation signed a statement last week that said the ban would interfere with an accused person’s constitutional right to defend themselves.
“We are absolutely opposed to the limitations of defenses,” said Alice Fontier, the director of the criminal defense practice of the Bronx Defenders, which signed the statement. “It’s ultimately about due process and a fair trial to anyone that comes before the court.”
Mr. O’Donnell, a former criminal defense lawyer, said he understood those concerns, but believed that New York needed to pass the law to further protect L.G.B.T. victims.
“In the end, our state needs to stand up and say being gay, being trans, is not a defense for killing someone,” he said.
The panic defenses stemmed from psychologists’ assertions that same-sex attraction or gender dysphoria were mental illnesses, according to a 2016 study by the Williams Institute at the U.C.L.A. School of Law,.
Those notions were discredited by the medical community in the 1970s, but not before defendants began to argue that upon learning a victim was gay, lesbian, bisexual or transgender, they suffered temporary insanity that spurred their violent actions.
The defenses effectively shifted blame onto the victim, re-victimizing them, according to Richard Saenz, a lawyer at Lambda Legal, a national L.G.B.T. civil rights organization.
“It assumes that this person was hiding or trying to be deceptive in some way,” Mr. Saenz said. “And when their sexual orientation or gender identity was discovered, the response was reasonable, even to the point of death.”
The defense strategy received widespread attention after Matthew Shepard, a gay college student, was killed in Wyoming in 1998. Lawyers for one of the accused men tried to argue that their client had beaten Mr. Shepard to death after Mr. Shepard made unwanted sexual advances on him.
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A judge rejected the argument, but the conversation spilled from a Wyoming district court to the national spotlight.
In New York, one of the most high-profile cases involving the defense came in 2013, after a transgender woman, Islan Nettles was beaten to death on a street in Harlem. Her attacker told the police he had flown into “a fury” after finding out that Ms. Nettles, with whom he had been flirting, was transgender.
The attacker, James Dixon, ultimately pleaded guilty to manslaughter and received 12 years in prison, a sentence that Ms. Nettles’s family and activists said would have been harsher had he not been able to cite “transgender panic” in his confession.
The American Bar Association formally called on governments to end the use of panic defenses in 2013. California was the first state to ban the defenses, in 2014. Illinois followed in 2017, and Rhode Island the year after.
Efforts seemed to have picked up this year. Nevada banned the defenses in May, and Hawaii and Connecticut have sent similar bills to their governors, neither of whom have signed them. (A spokeswoman for Gov. Ned Lamont of Connecticut said he planned to do so. A spokeswoman for Gov. David Ige of Hawaii said he was still reviewing the legislation.)
In Congress, Senator Edward J. Markey and Representative Joseph P. Kennedy III, both Massachusetts Democrats, introduced bills last year and this year that proposed to ban the defenses in federal courts.
In New York, Mr. Cuomo’s efforts came as part of a push to advance gay and transgender rights. In January, at the start of the legislative session, New York banned “conversion therapy” for minors, in which mental health professionals work to change a child’s sexual orientation or gender identity.
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The same month, the Legislature also passed a bill protecting transgender and gender nonconforming people under New York’s discrimination and hate crimes laws.
Mr. Cuomo had originally proposed ending the panic defense as part of his executive budget. In the last month, he made the issue a priority, holding a rally last week with the Bravo TV personality Andy Cohen.
“With the enactment of this measure, we are sending a noxious legal defense strategy to the dustbin of history where it belongs,” Mr. Cuomo said in a statement on Wednesday.
Opponents of New York’s panic defense bill were quick to applaud lawmakers’ strides toward securing L.G.B.T. rights, and said they found the defenses problematic.
“I don’t think that the homophobia or transphobia is acceptable,” said Lori Cohen, the president of the New York State Association of Criminal Defense Lawyers, which signed the statement opposing the legislation.
But Ms. Cohen, who identified as a lesbian, said she was skeptical that a measure would effectively address violence against gay and transgender people.
Instead, she said, lawmakers should address the underlying social issues that led judges or juries to accept these defenses.
Mr. Hoylman rejected that argument.
“I don’t think we can leave it to judges and juries given the record of homophobia that we’ve seen in courtrooms,” he said. “We’re acting prudently to codify values of tolerance and acceptance of L.G.B.T.Q. people.”
A version of this article appears in print on June 20, 2019, on Page A28 of the New York edition with the headline: Rejecting a Murder Defense.