Following the scientific revolution, being a natural scientist meant quantitatively measuring one’s subject matter, and ideally, performing experiments. In the nineteenth century, experimental and psychometric methods came into existence.

The first experimental technique to appear was mental chronometry, measuring the speed of mental processes. On the brain side, the premier physiologist of the nineteenth century (with whom Wundt studied), Hermann von Helmholtz (1821-1894), measured the speed of neural transmission. On the mental side, astronomers developed mental chronometry to solve a disturbing problem. Before photography, astronomers mapped the stars by noting the exact time (registered by the ticks of a special clock) at which a star crossed their telescope’s reticle. Unfortunately, two astronomers making the same observation at the same time often disagreed about the moment of transit, throwing doubt on the accuracy of star maps. The German astronomer Friedrich Bessel (1784-1846) studied these differences in judgment time, hoping to reconcile different astronomers’ observations by means of “personal equations” comparing them. The Dutch physiologist F. C. Donders (1818-1889) then developed a “subtractive method” for measuring inner mental processes. A task such as responding differently to two lights could be analyzed as a compound of a simple reaction, the response to the light, and a preceding judgment, or discrimination, of which light had occurred. The judgment time could then be measured indirectly by subtracting the time for a simple response to one light from the (longer) time taken to respond to two. Chronoscopes, special clocks for finely measuring reaction times, were mainstays of early laboratories, and mental chronometry is still widely used in cognitive psychology.

Some historians date the inception of experimental psychology to 1860, when Gustav Theodor Fechner (1801-1887) published Elements of Psychophysics. Fech­ner was a somewhat eccentric physicist at Leipzig (where Wundt founded the first laboratory) who became concerned with mathematically demonstrating the correspondence between mind and body. He developed sophisticated methods for precisely measuring the conscious sensation resulting from a stimulus of known value. For example, subjects might discriminate among varying weights, and these judgments could then be mapped onto the objective differences in the values of the weights. Out of this work came something Kant said was impossible, the first psychological law, Fechner’s law, that the strength of a sensation (S) is a logarithmic function of the strength of the stimulus (R. for Reiz), multiplied by a constant: S = k log R.

Psychophysical scaling, like mental chronometry, was a staple of the early laboratories and remains in use today, for example, in measuring pain. But it had a more general importance for the founding of psychology. Following Descartes, philosophers had introspected their minds, but never settled such seemingly straightforward questions as how many ideas consciousness held at a single time. Led by Wundt, scientific psychologists saw that the failure of philosophical or armchair introspection lay in its lack of methodical control. Psychophysics provided a model for making introspection scientific: expose subjects (originally called observers of consciousness) to known stimuli that could be systematically varied and collect from them simple reports on what they found in consciousness. Thus, one might present subjects with simple arrays of letters under varying conditions (e.g.. different exposure times. different number or physical arrangements of letters), and ask them to report how many they could see. Unlike results from philosophers’ armchairs, results from laboratory experiments were quantitative and reliable. Scientific psychology was underway.

The companion to experimentation was psychometrics, measuring differences in mental attributes by means of a mental test. a term coined in 1890 by American psychologist James McKeen Cattell (1860-1944). Various nineteenth-century psychologists developed techniques of mental testing, but the most important—certainly in the English-speaking world—was Sir Francis Galton (1822-1911), with whom Cattell studied after taking his degree with Wundt. A cousin of Darwin’s, Galton was interested in the evolution of mental abilities, chiefly intelligence. He sought ways to measure intelligence and pioneered statistical methods for treating psychometric data. For example, he invented the correlation coefficient to determine if schoolchildren who do well in one subject do well in others, and he established an anthropometric laboratory for testing people on a wide variety of capacities. Although Galton pioneered mental testing, his own tests proved to be of little value, and were replaced by sounder ones, such as Alfred Binet’s (1857-1911) test of intelligence.

Galton also pioneered the application of psychometrics to social issues. He believed that intelligence and other mental traits were highly heritable and that the intelligence of Britons was declining. To remedy the situation, Galton proposed schemes of eugenics, selectively breeding people for high intelligence, the way horses are selectively bred for speed. Although ignored at first, eugenics was later practiced (in ways Galton would have rejected) in the twentieth century by countries as diverse as Nazi Germany, socialist Sweden, and individualistic America, causing human misery and scientific and social disputes.

Mental testing changed psychology. It broadened the scope of psychology to include topics such as intelligence and personality that lay outside introspective and experimental reach. Mental testing also pushed psy­chology in applied directions, away from the pure re­search of the German laboratories. The field of clinical psychology began in 1896 with the establishment of a “psychological clinic” at the University of Pennsylvania by Wundt’s student, Lightner Witmer (1867-1956). The clinic tested the mental abilities of children referred by the Philadelphia school system. Similarly, psychology of business commenced with Walter Dill Scott’s (1869­1955) use of mental tests for employee selection. Mental testing also began a profound alteration of psychology’s subject matter. Although they recorded behavior—a keypress, a verbal report—experimentalists were really interested in getting at subjective states of consciousness. Mental testing, however, was a more thoroughly objective affair. An intelligence quotient was not an introspective report of a private conscious fact, but was a summary record of success and failure on a test. a fact in its own right. Mental testing thus represented a step toward defining psychology as a science of behavior, the study of what people do, rather than what they experience.