By 1987, 120 men and women had agreed to donate their brains after death. They all submitted to thorough psychological and intelligence tests so that each brain would be accompanied by a detailed profile of the mind that had animated it.

In the prime of life, the cerebral cortex contains 25 billion neurons linked through 164 trillion synapses.

Thoughts thread through 7.4 million miles of dendrite fibers and 62,000 miles of axons so compacted that the entire neural network is no larger than a coconut.

No two brains are identical, nor are two minds ever the same.

With so many well-documented donors, however, Witelson could conduct comparative brain studies on an unprecedented scale.

She could confidently seek relationships between anatomical features and mental capacities. She could also compare right-handers and left-handers, and sort the organs by gender.

In an era when people probe the thought process with scanners, radioactive tracers and super-conducting sensors, Witelson's approach was deliberately old-fashioned.

She measured her brains.

She weighed them.

She cut them up and counted the cells.

She traced synapses, the junctures where impulses pass from one neuron to another in the hidden root cellars of the brain.

Wherever she looked, she discerned subtle patterns that only gender seemed to explain.

"We actually didn't set out to find sex differences," she said. "Sometimes as a scientist, you are doing one thing and you bump into something else."

Controversial Matters

The brains in Witelson's freezer are contested terrain in a controversy over gender equality and mental performance.

Her findings — published in Science, the New England Journal of Medicine, the Lancet and other peer-reviewed journals — buttress the proposition that basic mental differences between men and women stem in part from physical differences in the brain.

Witelson is convinced that gender shapes the anatomy of male and female brains in separate but equal ways beginning at birth.

On average, she said, the brains of women and men are neither better nor worse, but they are measurably different.

Men's brains, for instance, are typically bigger — but on the whole, no smarter.

"What is astonishing to me," Witelson said, "is that it is so obvious that there are sex differences in the brain and these are likely to be translated into some cognitive differences, because the brain helps us think and feel and move and act.