Starch Professor of Psychology Jerome Kagan, a pioneering scholar of child psychology and teacher of the popular Social Analysis course, "Children and their Social Worlds," has announced he will retire from teaching at the end of the semester.

"I'll be 71 and there's a nice cycle to life," he said in an interview last night. "I think its time for younger people to move into professorship positions."

The psychology department has tenured two new professors who will join its Faculty next year.

Kagan said he plans to keep his laboratory and will serve as a research professor.

Child psychology has developed rapidly since Kagan came to Harvard in 1964. Then, several key assumptions governed study in the field. Psychologist's assumed, for instance, that the brain's development was fixed at birth. It was also thought that family relationships are wholly important in predicting the future social behavior of children. Experts in the field also assumed that culture and class had little affect on a child's chances for happiness and fulfillment.

"In the early years, there was a deep belief that what happened to the child in the early years of life fixed the child's development in a major way. We know know that's not true," Kagan said.

And only very recently have scientists realized the amazing plasticity of children's brains lasts even into adolescence.

"We now understand...in a deeper sense the correlation between maturation of the brain and the appearance of the universal milestone of development," Kagan said.

Kagan's work has emphasized the importance of recognizing the unique biology of children, which contributes in unusual ways to their development and maturation.

"In 1964, I never would have entertained the idea that the genetically based temperament of the child would play such a powerful role in determining personality traits," he said.

Eschewing relativists in his own field, Kagan now believes that fundamental notions of right and wrong are suggested to us by our biology, so, at an early age, children can benefit from instruction in ethics.

Most recently, he and a team of graduate students have studied shyness.

His research has led him to the conclusion that children who are socially anxious at young ages often become shy adolescents.

Kagan found biological differences in extremely shy children, leading his team of scientists to develop a theory about why children are unusually shy.

"Most of the milestones of maturation are biological," Kagan said. "In the opening year, the growth of the human brain does constrain and modulate when language appears, when guilt appears, and when major emotions appear."

Kagan's books have made the bestseller lists, owing to his status as a popularizer of contemporary child psychology.

His more technical scholarship is a staple of child psychology courses around the country.

Kagan is the second tenured Harvard psychologist to announce that he will leave the department this year.

Earlier this fall, Lindsey Professor of Psychology Sheldon H. White '50-'51 announced his retirement. He will depart from Harvard after at the conclusion of the 2000-2001 school year.

To replace the two departures, the department has hired two cognitive neuroscientists.

Susan E. Carey '64 and Elizabeth S. Spelke '71 both accepted tenure here last week.

"We needed to recruit the two best senior people, and these are the two we came up with," said Daniel L. Schacter, chair of the psychology department.

Spelke is currently a professor in the Department of Brain and Cognitive Sciences at MIT. She received her PhD from Cornell.

With Harvard professor Marc D. Hauser, she is studying infant cognition.

Spelke's experimental methods have opened up whole new ways of studying infant thought that were not available before, Schacter said.

Carey is currently a professor of psychology at New York University, where she studies the origins of how humans learn.

According Schacter, both women are at the top of their field.

"Both are international leaders in developmental psychology and have done pioneering work on infant and child cognitive science," he said. "They have set the international standard...and we are fortunate to have them here."

Schacter said he expected Carey and Spelke to ably lead College classrooms.

"Both are outstanding lecturers and will be outstanding teachers," he said. "They will teach a mixture of undergraduate and graduate classes and a mix of lectures and seminars."

Because both have run active labs and are actively involved in research, their arrival here will open new opportunities to get both graduate students and undergraduates involved in cutting edge research, Schacter said.

"Having two people whose work compliments each other's will make this an intellectually exciting time here in developmental psychology," he said.

According to Spelke, her decision last week to accept tenure here is partly based on the fact that she will be able to continue her research. She said she is eager to collaborate with Carey on new research.

"The psychology department at Harvard is an extremely exciting place now, and to join with Susan Carey is a great opportunity," Spelke said.

--Marc J. Ambinder contributed to the reporting and writing of this story.