The second quantitative breakthrough would be the measurement of something far more complex, far more "psychological:" intelligence. We owe this to two great minds in particular: Sir Francis Galton in England and Alfred Binet in France.

Francis Galton was born February 16, 1822 near Birmingham, England. He was the youngest of 7 children, and first cousin of Charles Darwin. His father, a wealthy banker, insisted on educating Francis at home, especially considering that Francis could read at 2 and a half years old!

Later in childhood, he was sent off to boarding school, which he despised and criticized even in adulthood. At 16, he went to medical school at King’s College at Oxford. He finished his degree at Cambridge in 1843, at 21.

His father died, leaving Galton a wealthy young aristocrat. He traveled extensively and became a member of the Royal Geographical Society, for which he developed maps of new territories and accounts of his adventures. He became president of that organization in 1856.

Galton had a penchant for measuring everything -- extending even to the behinds of women he encountered in his travels in Africa (something he had to do from a distance, of course, by means of triangulation). This interest in measurement led to his invention of the weathermap (including highs, lows, and front -- terms he introduced), and to suggesting the use of fingerprints to Scotland Yard.

His obsession eventually led to his efforts at measuring intelligence.  In 1869, he published Hereditary Genius:  An Inquiry into its Laws and Consequences, in which he demonstrates that the children of geniuses tend to be geniuses themselves.

In 1874, he produced English Men of Science:  Their Nature and Nurture, based on long surveys passed out to thousands of established scientists.  In this volume, he noted that, although the potential for high intelligence is still clearly inherited, it also needed to be nurtured to come to full fruition.  In particular, the broad, liberal education provided by the Scottish school system proved far superior to the English school system he hated so much.

In 1883, he wrote Inquiries into Human Faculty and its Development.  This would be the first time anyone compared identical and fraternal twins, a method now considered ideal when investigating nature vs nurture issues.

In 1888, he published Co-Relations and Their Measurement, Chiefly from Anthropometric Data.  As the title suggests, it was Galton who invented correlation, as well as scatter plots and regression toward the mean.  Later, Karl Pearson (1857-1936) would discover the mathematical formulation of correlation.

Sir Francis died in 1911, after an incredibly productive, if somewhat eccentric, life.