Studies suggest that children born into smaller families, regardless of income and other factors, tend to attain higher intellectual achievement than do children in larger families, several social scientists said today.

Dr. Robert B. Zajonc of the University of Michigan said new research indicates that children with one brother or sister perform better in school and on standardized tests than do children with more than one sibling. The firstborn has the intellectual advantage in the two-child family, he said.

"If you ask me the question, 'what is the family configuration that rewards the highest scores and intellectual performance,' it's a two-child family with a spacing of more than two years," he said.

Zajonc cautioned parents against "making decisions on this alone," however. He noted that "we know very little about how family size affects personality factors," such as sociability, resilience or the absence of anxiety.

"If we only pay attention to IQ, we may have people who have somewhat higher IQ but at the same time are very selfish or very unsociable."

Zajonc spoke today at a panel and news conference at the annual meeting here of the American Association for the Advancement of Science, the nation's largest general scientific organization.

Other scientists participating in the symposium agreed that family size is important in intellectual and educational performance, but they disagreed on the causes and implications of research results.

Dr. Judith Blake of the University of California at Los Angeles said the differences between small and large families are significant enough to affect decisions about how many children to have.

"The data I have indicates the advantages of coming from a small family are gigantic," Blake said. "By that, I mean two or three . . . . The disadvantages of coming from a six- or seven-child family are enormous. They are a great drag on a person's educational advancement."

The greatest hazard for children in large families is dropping out of school, Blake said. After accounting for family background, she found that those from large families "lose about a year of graded schooling on average, which translates into large differences in proportions graduating from high school."

Zajonc said he believes that although intellectual differences between children may have minimal impact on individual families, family size may have substantial national consequences.

His research found "a decline in performance for each additional child that is very systematic" in standardized test scores. He presented data showing that changes in U.S. family size may account in part, for example, for recent fluctuations in high school students' scores on the Scholastic Aptitude Test (SAT).

He said scores dropped from 1963 to 1980 as the baby-boom generation took the tests. He predicted that such scores will continue their recent rise until the turn of the century, but that because of a new rise in birth rates after 1980 a decline in scores will recur early in the next century when today's youngsters take the SATs.

Paul Taubman of the University of Pennsylvania said a new study there of 4,000 children has found that birth order and family size have an effect on a child's education, after the parents' ages, income and education are taken into account.

"Family size may be important because if more children have to share parental financial, emotional and time resources, each child may get less," Taubman said. His research also found the firstborn faring best.

Michigan State University biologist James Higgins countered that heredity is far more important in a child's intellectual achievement than the number, order or spacing between children.