Francis Galton as Differential Psychologist:

Galton was one of the first experimental psychologists, and the founder of the field of enquiry now called Differential Psychology, which concerns itself with psychological differences between people, rather than on common traits.  He started virtually from scratch, and had to invent the major tools he required, right down to the statistical methods - correlation and regression - which he later developed.  These are now the nuts-and-bolts of the empirical human sciences, but were unknown in his time.  One of the principal obstacles he had to overcome was the treatment of differences on measures as measurement error, rather than as natural variability.

His influential study Hereditary Genius (1869) was the first systematic attempt to investigate the effect of heredity on intellectual abilities, and was notable for its use of the bell-shaped Normal Distribution, then called the "Law of Errors", to describe differences in intellectual ability, and its use of pedigree analysis to determine hereditary effects. 

Later Galton went on to suggest the use of twin studies to disentangle nature from nurture, by comparing identical twins to fraternal twins.  The research program that Galton initiated in this regard has developed into the important field of behaviour genetics.

Galton later broadened his study of human traits into general anthropometry, or "measurement of man", trying to find as many measurable traits as possible, so that their distribution and heritability could be determined. 

His psychological studies also embraced mental differences in visualization, and he was the first to identify and study "number forms", now called "synaesthesia".  He also invented the word-association test, and investigated the operations of the sub-conscious mind.  His work in this area was collected into a wide-ranging volume called Inquiries into Human Faculty, which must be read today with Galton's broader research program in mind: to identify and measure variable human traits.

Inheritance of ability

Facsimile Paper 1865 'Hereditary talent and character.' Macmillan's Magazine 12 : 157-66, 318-27
Facsimile Letter 1868 'Hereditary genius.' [Letter] Notes and Queries on China and Japan 2 (1, January)
Facsimile Paper 1869 Hereditary Genius: the Judges of England between 1660 and 1865. Macmillan's Magazine : 424-431

General Psychology

Facsimile Paper 1871 Gregariousness in cattle and in men. Macmillan's Magazine 23 : 353-57
  Paper 1877 'La psychophysique' La Revue Scientifique 13 : 494-8
Facsimile Paper 1880 'Mental imagery' Fortnightly Review 28 : 312-24
Facsimile Paper 1880 'Visualised numerals.' Nature 21 : 252-6, 494-5
Facsimile Letter 1880 'Visualised numerals.' [Letter] Nature 21 : 323
Facsimile Paper 1881 'The visions of sane persons.' Fortnightly Review 29 : 729-40
Facsimile Paper 1881 'Visualised numerals.' Journal of the Anthropological Institute 10 : 85-102
Facsimile Paper 1881 'The visions of sane persons.' [with slight variations] Proceedings of the Royal Institution 9 (May 13) : 644-55
Facsimile Paper 1884 'Free-will-observations and inferences.' Mind 9 : 406-13
Facsimile Paper 1886 Supplementary notes on "Prehension in idiots". Mind 12 : 79-82
Facsimile Letter 1887 'Thoughts without words.' [Letters] Nature 36 : 28-9, 100-1
Facsimile Letter 1887 Thoughts without Words [Letters] Open Court 1 : 441-4, 472-4, 498-50
Facsimile Paper 1891 [Discussion on 'An apparent paradox in mental evolution', Lady Welby] Journal of the Anthropological Institute 20 : 304-23
Facsimile Review 1894 'Psychology of mental arithmeticians and blindfold chess-players.' [Review of Psychologic des Grands Calculateurs et Joueurs d'Echecs, Alfred Binet] Nature 51 : 73-4
Facsimile Paper 1894 'Arithmetic by smell.' Psychological Review 1 : 61-2
Facsimile Review 1895 'Personality,' [Review of The Diseases of Personality, Th. Ribot] Nature 52 : 517-8
Facsimile Paper 1896 'A curious idiosyncrasy.' Nature 54 : 76
Facsimile Review 1898 'Evolution of the moral instinct.' [Review of The Origin and Growth of the Moral Instinct, Alexander Sutherland] (Signed F.G.). Nature 58 : 241-2


Facsimile Paper 1879 'Psychometric experiments.' Brain 2 : 149-62
Facsimile Paper 1879 'Psychometric facts.' Nineteenth Century (March) : 425-33
Facsimile Paper 1880 'Statistics of mental imagery.' Mind 5 : 301-18
Facsimile Paper 1884 'Measurement of character.' Fortnightly Review 36 : 179-85
Facsimile Report 1884 The Measurement of Human Faculty' (Rede Lecture) Nature 31
Facsimile Paper 1889 'An instrument for measuring reaction time.' Report of the British Association for the Advancement of Science 59 : 784-5
Facsimile Paper 1890 'Exhibition of instruments (1) for testing the perception of differences of tint, and (2) for determining reaction-time.' Journal of the Anthropological Institute 19 : 27-9
Facsimile Paper 1890 [Remarks following 'Mental tests and measurements', J. McK. Cattell] Mind 15 : 380-1
Facsimile Paper 1893 'Measure of the imagination.' Nature 47 : 319-21
Facsimile Paper 1893 'The just-perceptible difference.' Proceedings of the Royal Institution 14 : 13-26