The Supreme Court, striking down part of a Maryland victims'-rights law, ruled 5 to 4 yesterday that emotional statements about a murder's effect on the victim's family unduly inflame a jury that is weighing the death penalty.

Saying that the use of such "victim-impact statements" in capital cases violates a defendant's constitutional rights, Justice Lewis F. Powell Jr. ruled that "any decision to impose the death sentence must be, and appear to be, based on reason, rather than caprice or emotion."

The ruling is the high court's first involving victims'-rights laws recently passed in 36 states.

Powell, joined by Justices William J. Brennan Jr., Thurgood Marshall, Harry A. Blackmun and John Paul Stevens, said the court was not deciding whether victim-impact statements are constitutional in noncapital cases.

The ruling, which involved the brutal 1983 murder of an elderly couple in East Baltimore, was denounced by victims'-rights groups that had pushed for passage of the Maryland law.

E. Gene Patterson, executive director of a clearinghouse for many such groups, said the ruling was "quite a blow to the victims'-rights movement" and a "slap in the face to all victims of violent crime.

"In essence, it says the rights of convicted murderers take precedence over the rights of innocent victims," Patterson said.

Maryland Sen. Howard A. Denis (R-Montgomery), a cosponsor of the state's initial victim-impact bill, called the ruling "a setback" but said it was written "in a narrow way so that prosecutors can still use the victim-impact statements if they opt for a life sentence instead of the death penalty."

George E. Burns Jr., an attorney for one of the convicted murderers, said the decision "shows that the Supreme Court has a continuing concern that you exercise control and care in death sentences and that it is not 'anything goes' " in capital cases. Burns said the decision would affect about a half dozen cases in Maryland, which has invoked its new law more often than other states have used theirs.

Yesterday's ruling involved the 1983 murders of Irvin Bronstein, 78, and his wife, Rose, 75. The murderers, John Booth and Willie Reid, went into the Bronstein's home to steal money to buy heroin.

Booth, a neighbor, knew that the couple could identify him. The Bronsteins were bound and gagged and stabbed repeatedly in the chest with a kitchen knife. Later that night, the murderers and two friends returned to the home to steal more property. The bodies were discovered two days later by the Bronsteins' son.

After the jury convicted Booth, the prosecution read a victim-impact statement that included statements by the couple's children and grandchildren. In one statement, the son said he suffered from lack of sleep and depression after his parents were "butchered like animals."

Prosecutors argued that the crime's effect on its victims was relevant to sentencing and that society's valid aim of retribution or revenge is furthered by having the sentencing jury or judge consider the impact of the crime.

The trial judge and the Maryland Court of Appeals agreed that the use of such statements is constitutional, but the Supreme Court overturned the decision.

"One can understand the grief and anger of the family caused by the brutal murders in this case," Powell said, "and there is no doubt that jurors generally are aware of these feelings. But the formal presentation of this information by the state can serve no other purpose than to inflame the jury and divert it from deciding {the sentence} on the relevant evidence."

Powell said that admitting "these emotionally charged opinions as to what conclusions the jury should draw from the evidence is clearly inconsistent with the reasoned decision-making we require in capital cases."

"Although this court will normally defer to a state legislature's determination of what factors are relevant to the sentencing decision," Powell said, "the Constitution places some limits on this discretion."

Powell said the "emotional impact of the crimes on the family" and the family's opinions about the crimes and the defendant were "irrelevant to a capital sentencing decision. Allowing the jury to consider such statements "creates a constitutionally unacceptable risk that the jury may impose the death penalty in an arbitrary and capricious manner," he said, in violation of the Eighth Amendment's ban on "cruel and unusual punishments."

It also might encourage the death sentence for murderers of victims whose "family members were articulate and persuasive" but not for those whose relatives were less well-spoken. "Certainly the degree to which a family is willing and able to express its grief," Powell said, "is irrelevant to the decision whether a defendant, who may merit the death penalty, should live or die."

Justice Byron R. White, in dissent, said "the affront to humanity of a brutal murder such as {Booth} committed is not limited to its impact on the victim or victims; a victim's community is also injured, and in particular, the victim's family suffers shock and grief of a kind difficult even to imagine for those who have not shared a similar loss."

White was joined by Chief Justice William H. Rehnquist and Justices Sandra Day O'Connor and Antonin Scalia. "I cannot agree," White said, "that there was anything 'cruel and unusual' or otherwise unconstitutional about the legislature's decision to use victim-impact statements in capital sentencing hearings."

"If anything," he said, "I would think that victim-impact statements are particularly appropriate evidence" in death-penalty cases, which allow the defendant to present mitigating evidence during sentencing.