Darwin explores the biological aspects of emotional behaviour and the animal origins of human characteristics like smiling and frowning, shrugging shoulders, lifting eyebrows in surprise, and baring teeth in an angry sneer.

Darwin sought out the opinions of some leading psychiatrists, notably James Crichton-Browne, in preparation for the book, which forms his main contribution to psychology.

[5] The book involves several innovations: Darwin circulated a questionnaire (probably inspired by his cousin, Francis Galton)[citation needed] during his preparatory research; simple psychology experiments on the recognition of emotions with his friends and family;[6] and (like Duchenne de Boulogne, a physician at the Salpêtrière Hospital) the use of photography in his presentation of scientific information.

[9] Darwin fully grasps his conception of natural selection towards the end of September 1838, after encountering the sixth edition of Essay on Population (1826) by Thomas Malthus.

On 21 September 1838, Notebook M discloses a "confusing" dream where Darwin found himself involved in a public execution; the corpse had come to life and joked about not running away and facing death like a hero.

A detailed discussion of the significance of Notebook M can be found in Paul H. Barrett's Metaphysics, Materialism and the Evolution of Mind – Early Writings of Charles Darwin (1980).

In its public management, Darwin understood that his evolutionary theory's relevance to human emotional life could draw an anxious and hostile response.

While preparing the text of The Variation of Animals and Plants Under Domestication in 1866, Darwin began to explore topics related to human ancestry, sexual selection, and emotional life.

After his initial correspondence with the psychiatrist James Crichton-Browne,[14] Darwin set aside his material concerning emotional expression to complete Descent of Man, which covered human ancestry and sexual selection.

Darwin brings his evolutionary theory close to behavioural science in Expression, although several commentators have perceived a spectral Lamarckism within its text.

Darwin displayed several biographical links between his psychological life and locomotion: taking long, solitary walks around Shrewsbury after his mother died in 1817; in his seashore rambles near Edinburgh with the Lamarckian evolutionist Robert Edmond Grant in 1826 and 1827;[16][17][18] and in laying out the sandwalk, his "thinking path", at Down House in Kent in 1846.

Darwin's response to Bell's natural theology is discussed in Lucy Hartley's Physiognomy and the Meaning of Expression in Nineteenth Century Culture (2001).

Darwin corresponded with James Crichton-Browne, the son of the phrenologist William A. F. Browne and the then medical director of West Riding Pauper Lunatic Asylum.

"[24] Darwin also drew on his personal experience of the symptoms of bereavement and studied the text of Henry Maudsley's 1870 Gulstonian Lectures on Body And Mind.

Darwin closes the book with chapter 14, where he summarises his central argument, demonstrating how human emotions link mental states with bodily movement.

Robert F. Cooke, John Murray III's cousin and partner in his publishing company, expressed concern about the increasing production costs and warned Darwin that including the photographs "will make a terrible hole in the profits of each edition".

The proofs, tackled by his daughter Henrietta ("Ettie") and son Leo, required a major revision, which made Darwin "sick of the subject and myself, and the world".

[31] Expression was published by John Murray on 26 November 1872 as a sequel to The Descent of Man and was expected to reach a broad audience in mid-Victorian England.

[34] A review in the January 1873 Quarterly Journal of Science concluded that "although some parts are a little tedious, from the amount of minute detail required, there is throughout so much of acute observation and amusing anecdote as to render it perhaps more attractive to general readers than any of Mr. Darwin's previous work".

[35] On 24 January 1895, James Crichton-Browne delivered the lecture "On Emotional Expression" in Dumfries, Scotland, presenting some of his reservations about Darwin's views.

[39] All these sensations and innervations belong to the field of The Expression of the Emotions, which, as Darwin (1872) has taught us, consists of actions which originally had a meaning and served a purpose.

[citation needed] The detailed approach to illustrating biological subjects[45] continued in contributions from various authors: photographer Eadweard Muybridge's work on animal locomotion,[46][47] which influenced the development of cinematography; Scottish naturalist James Bell Pettigrew's studies on animal locomotion, documented in his works Animal Locomotion, or, Walking, Swimming and Flying, with a dissertation on Aeronautics (1874) and Design in Nature (1908); the illustrated and controversial works of evolutionary biologist Ernst Haeckel; and to a lesser extent, in On Growth and Form (1917) by D'Arcy Thompson.