The District of Columbia Court of Appeals formally abolished yesterday the traditional legal presumption in custody cases that a young child is better off with its mother.

Judge Julia Cooper Mack, writing a lengthy opinion for the court, said that "eminent child psychiatrists have demonstrated that what a child needs is not a mother, but someone who can provide 'mothering'... And the ability of a person to provide 'mothering' does not necessarily correspond to the gender of the parent or the biological relationship between adult and child."

The decision reinforces the principle that custody battles between natural parents be resolved solely in the best interest of the child, Mack said in the opinion.

Judith Areen, a professor of family law at the Georgetown Law Center, said the decision would apply to about 10 percent of the custody situations that eventually reach the courts in the District. The other 90 percent are resolved between the parents in favor of the mother, she said.

"The common wisdom in the District is if the father seeks eustody he has a 50-50 chance, assuming everything is equal," said Areen, who recently wrote a textbook on family law.

She quoted a recent nationwide survey as saying the presumption in favor of the mother was abolished this year in Maryland, but in Virginia, if the parents' situation is equal, the court favors the mother.

Rules like the presumption in favor of the mother are "ill-suited for determining the future of a unique being whose adjustment is vital to the welfare of future generations," Mack wrote in her 15-page majority decision.

"... (magic) formulas have no place in decisions designed to salvage human values," Mack said in the opinion.

The full nine-member court agreed that the custody case in question, involving a dispute between the natural parents of a 6-year-old girl, should be sent back to D.C. Superior Court for reconsideration. But in a lengthy dissent, three judges accused their colleagues of abolishing "a rule that did not actually exist in this jurisdiction."

Judge George R. Gallagher, joined by Judges Frank Q. Nebeker and Stanley S. Harris, noted in his dissent that the trial judges have broad discretion in deciding custody cases. The majority opinion, Gallagher said, "will have little or no effect on the future resolution of custody disputes in the real world of the trial court -- or so it seems to me."

The established principle in the District of Columbia is that custody disputes be resolved in the best interests of the child, Gallagher wrote. The presumption that favored the mother was not a rule, Gallagher argued, but rather an inference -- drawn from commonly accepted human experiences -- that a trial judge could use in balancing factors in a case.

Gallagher said that the majority opinion does not preclude trial judges from considering certain "realities" if the situation of both parents is equal. Gallagher cited a portion of a brief filed in the case that said: "There are, after all, certain biological and physiological differences between male and female which will withstand all assaults of modern day mores."

Richard S. Bromberg, an attorney who represented the mother in the case before the Appellate Court, characterized the majority decision as "the final blow" to the so-called "tender years doctrine" here, which favored mothers in custody cases. But the doctrine, Bromberg agreed," was pretty well dying."

Despite the Appellate court decision, Bromberg said, "judges are still going to be able to favor whomever they want to as long as they couch their order in terms of the best interests of the child.

Areen said the decision would be "helpful" in clearing up questions in other cases in the local courts.

But, Areen said, although the doctrine has been abandoned, "who can really control the heart and mind of an individual judge?"

She noted that the U.S. Supreme Court has on several occasions declined to decide the issue.

The case before the appellate court involved a daughter born in 1972 to unmarried high school students, both 16-year-olds and living with their parents. The child lived with her mother for two years and was then voluntarily given into the custody of her father, Mack wrote in her opinion.

Two and one-half years later, the mother went to Superior Court to regain custody of her child.

There Judge Eugene N. Hamilton ruled that the child be returned to the mother, based on the legal presumption that the child belongs with its mother if there is no showing that she is unfit to tend to the child's welfare. Without that showing, Hamilton said, he was compelled to favor the mother and need not go on to the question of the child's best interests -- a ruling the appellate court said was clearly wrong.

Mack was joined in her opinion by Chief Judge Theodore R. Newman Jr. and associate Judges John Ferren, Catherine B. Kelly, John W. Kern III and J. Walter Yeagley.

The presumption in favor of the mother became a "crutch" used by judges in difficult custody cases, the court's opinion said. The suggestion that a mother must be shown to be unfit only worsens the relationship between parents "to the detriment of the child," Mack said in a footnote to her opinion.

The "delicate question" of the child's best interests, Mack said, should be made by the trial judges on the facts relating to the child alone.