An initial question must be, ‘What is a portrait’? According to Ernst Van Alphen an artistic portrait – as opposed to a portrait used for medical or police documentation,
“...is more than a documentation. The portrayer proves his/her artistic originality by consolidating the self of the portrayed. Although the portrait refers to an original self already present, this self needs its portrayal in order to secure its own being. The portrayer has enriched the interiority of the portrayed self by bestowing exterior form of it. For, without outer form the uniqueness of the subject’s essence could be doubted. The portrayer proofs his/her own uniqueness by providing his proof.” 
From the above definitions, we could deduce that a successful portrait should represent a sort of “hidden uniqueness” which the subject possesses in such a way that this uniqueness can also be clearly read by the viewer. It is also suggested that the portrait demonstrates the unique artistic ability of the photographer to capture the sitter’s true appearance. The viewer then will be confronted simultaneously with the essence of the sitter and the photographer, in a process that could be expressed as “the meeting of two subjectivities” .
In his article Truth and Method, H. G Gadamer suggests that in order to produce a successful portrait, where “an individual is not represented idealised, nor in an incidental moment, but in the essential quality of his true appearance”, the portrait must “bring in two referents: the portrait as a body – as a material form- and the essence of the sitter – his or her unique authenticity ”.  Like Ernst Van Alphen, Gadamer insists on the need to prove the “uniqueness” of the portrayed in order to produce what he considers a successful portrait.
"La Maja Desnuda". Francisco Goya, between 17890 - 1800
The difference between portraits created in these various media appears straightforward: what photography adds to the portrait as opposed to other media is the absolute certainty that what is being shown once existed in front of the camera. On the other hand, if we wanted to be certain about the presence of the sitter who is painted or sculpted, we would probably need to verify testimonies from several witnesses. This is because the source of representation could well have been a previously made one (that is, for example, a pre-existing sculpture of a person), or even a product of the artist’s imagination. Furthermore, while painting and sculpting can only be a process of pure interpretation – more or less accurate - of the model’s appearance, the camera can only act as a mirror of reality, leaving the photographer with choices that are only a matter of selecting the “decisive moment” as expressed by Cartier Bresson. Any other arrangements made by the photographer, such as the staging, or the choice of model’s clothing, can certainly add meaning to the representation, but by no means can alter the subject’s essence in its purest sense, that is, their anatomical and psychical nature. The challenge for the portrayer then, lays in the ability to choose that concrete instant in which they can be sure of communicating their intention, whatever this might be, in relation to the depicted model.
Boy and his dead brother. "Unkown Author" UK, around 1910
With the invention of photography in the early nineteenth century, portraiture as a genre was democratized. Most people from all social classes could afford to have a photograph taken at same point in their life. Family albums and the record of significant moments of a family’s history became common practice. Even the dead were often captured through the camera lens with some sort of hope of keeping their essence alive in the photographic print. With the advent of the dry-plate era and the development of lighter weight photographic equipment, photographers were not only able to cross all frontiers and show us the most beautiful places on earth, but also to portray people from all over the world, showing the western public how the inhabitants from newly colonised worlds looked and offering a priceless tool for anthropologists to study anatomical differences between ethnic groups. These photographers from the colonial period had no intention of communicating any sort of uniqueness in terms of the psychological aspect of the model. They documented these people in the same way - with the same system - that they had been using to document every known creature on earth. Placing often tools for measurement within the frame, the sitter was always presented as “the other”, that is, someone to look at as if they were very distant and far different from our “superior western race”.
1847 Daguerreotype of Astrualian Aborigians.
NGA: Douglas T. Kilbrun
MODERN PHOTOGRAPHY AND AUGUST SANDER: RELATIONSHIPS OF OBJECTIVE DISTANCE
The photographic medium had proved to be one of the most powerful – and accurate - ways of documenting our history and society. Photographers like August Sander played a definite role in establishing this. In his book Face of our time, first published in 1929, Sander developed a strict method of documenting German society, offering a clear record of western types in the early twentieth century. Made to hold a still, quasi-pictorial pose in front of the camera, usually looking directly into it, Sanders’ subjects depict the whole range of the German social scale. From farmers to bankers, children or mentally ill people, his work offers an invaluable document for historians and sociologists, a masterpiece, celebrated worldwide and defined by Walter Benjamin as more than just a picture book; a real “training manual”
Boxer. Paul Rodersten und Hein Heese,
Köln, 1928. August Sander
Sander’s style of portraiture shares characteristics with colonial photography since he is not looking for the true “appearance” of the sitter, but aims instead to produce a complete archive by recording the physical appearance of different German types. However, unlike ethnographical photography, Sanders makes photographs with the use of a highly elaborate photographic technique. He offers the viewer more than just an objective documentation of human types, by presenting at the same time a series of fine-art portraits to be enjoyed in a museum environment.
With the proliferation of 35mm cameras and the consequent mobility allowed by its lighter weight, photographers were more than ever invited to discover the world through their camera lens. Documentary photography acquired a prominent role not only through its journalistic use, but also in the art world. Silver gelatine prints showing everyday scenes of people all over the world became very popular and their collectable value reached hitherto unknown limits. Indeed, the exhibition “The Family of Man”, curated by Edward Steichen for the New York Museum of Modern Art in 1955, offered the public the possibility of looking at “the world” through over five hundred photographs, taken by photographers from eight different countries all with the same intention: to demonstrate that humanity is “one” and that all human beings, in spite of their flaws, are beautiful.
Exhibition "The family of Man" MOMA, NY. 1955
CONTEMPORARY PHOTOGRAPHY: RELATIONSHIPS OF IDENTIFICATION
Later, in 1972, MOMA exhibited the work of Diane Arbus. Whilst it seemed to share the creative approach and intention of photographers whose work was exhibited in “The Family of Man”, in fact, it communicated something totally opposed to the idea of humanity as “one”. Through a series of a hundred and twelve photographs, all taken by her and apparently similar to each other, that is, portraits that had the same feeling, Arbus presented a world of “freaks” living in New York in the sixties. Most of the models confronted the camera with a natural gaze, observing the viewer with total frankness and revealing no consciousness of their own strangeness. Diane’s work does not invite the public to identify with the subjects she portrayed, nor does it pretend to suggest any kind of compassion for the subject matter. Instead, her images seem to wish to demonstrate that humanity is diverse by nature and that only this diversity – and therefore uniqueness - can highlight the extraordinary condition of each human being. According to Julian Stallabrass, the aesthetic representation of Arbus’ work helped her to achieve her intentions. That is, in spite of the fact that,
“her pictures are centred and frontal, they are free of composition […]. Varied compositional means are matched to variable subject matter, the point being that “freaks” can be found as much among the “normal” as the marginal”. 
"Puerto Rican woman with a beauty mark, 1965" Diane Arbus
Her work certainly generated a strong fascination. She seemed to have discovered a hidden “sub world” that coexisted with the busy New York society of the sixties. Her images offered an exoticism carried by a sitter that was no longer living miles away from the western world, neither in the United States’ countryside, but in their very same city.
It is arguable that the fascination generated by Arbus’ work is produced by a “visual pleasure” that is common to all photographic portraiture. This is described by David Bate when he says,
“there is a pleasure involved in the very process of recognition, of recognizing...The photographic portrait may offer the image of someone that the viewer can identify with, being as like them. Even if this is simply a wish that can never be fulfilled, it nevertheless provides a visual satisfaction in the fantasy of identification”. 
Dianne Arbus. "Mexican Dwarf"1955
But of course, this sense of satisfaction could also be produced by a rejection of such identification: the viewer might prove his or her “superiority” by recognizing certain weaknesses in the sitter, which they are certain that they do not posses themselves. In other words, whether identification occurs on the basis of recognition or rejection, what emerges is the fact that human beings enjoy looking at the portrayed subject and - more or less consciously - project their own feelings into any representation of “the other”, filling in any missing information with their own imagination in order to achieve certain satisfaction, with its consequent visual pleasure.
Richard Avedon, Bill Curry, Drifter, Interstate 40, Yukon, Oklahoma, June 16, 1980.
Another example of this identification through rejection is Richard Avedon’s project “In the American West”. In this, Avedon depicts a series of labourers, drifters, prisoners and the inmates of metal hospitals “who displayed the marks of their labour – blood, grime, amputations - and the way that prolonged work had honed and distorted their bodies as tools, before an implied audience of urban cosmopolitan types”. According to John Rohrbach, the commissioner of this project, the subjects were “people whom many of us would prefer to step quietly around if encountered in life”. As with Arbus’ work, Avedon’s series suggests a satisfaction generated by the rejection of identification between the subject matter and its viewers. Using an 10”x8” view camera and setting up a white background with the help of two assistants, Avedon would guide the sitters in their movements - through instruction - until they reached the desired pose. The models had been carefully chosen in order to find “a face that can hold a [museum] wall”. But despite the many arrangements made by the photographer and his large production team, Avedon insists that,
“these disciplines, these strategies, this silent theatre, attempts to achieve an illusion: that everything embodied in the photograph simply happened, that the person in the portrait was always there, was never told to stand there, was never encouraged to hold his hands, and in the end was never in the presence of a photographer”. 
Richard Avedon at work "In the American West"
Avedon openly describes the elaborate processes required to produce his work, whilst at the same time he seems convinced that his portraits achieve a “spontaneous look”. Given this, it is not surprising that critical doubts have arisen as to whether he accomplishes his intentions in this project.
Different styles of photography encourage different modes of identification. In the late twentieth century, photographers like Céline van Balen, Thomas Ruff and Rineke Dijkstra, started to develop a style of portraiture which depicted people in uniform series, facing the camera and looking straight into the lens. Often using a common background, the differences between the images lay mainly in the physical and anatomical characteristics of the sitter.
This method of portraying is certainly reminiscent of ethnographic photography. Sharing a “neutral” aesthetic, these late twentieth century portraits and ethnographic photographs both seem to present a typology of human figures. But while ethnographic photography was clearly presenting the model as “the other” who needed to be classified according to certain anatomical characteristics, now, as Julian Stallabrass suggests, “there appears a combination of a deadpan ethnographic method applied to subjects who are not definitively marked off from the viewer”.
Portraits by Rineke Dijkstra. From the series "Hilton Head Island, USA" 1992
Some of the best known work, from the photographers described above, which functions in this way, is Rineke Dijkstra’s series depicting youths standing by the beach or in the middle of the forest. Dijkstra employs a uniform composition with constant artificial lighting in which all subjects look directly into the lens of a 5”x4” camera placed in a low position. She seems to intend to demonstrate the uniqueness of the sitters not through their gaze, but through their anatomical differences. The physical differences are magnified in the giant enlargement of the print, giving the sense of the subjects’ “super-real” presence. As Martha Rosler has stated, the basis of the identification with the subject lies in a “physiognomic fallacy, in which the face and body are seen as the expression of character” . What we read from their gaze has less importance, as the person’s true appearance is revealed through the unique texture of their skin or the concrete length of their extremities.
According to Dijkstra, it essential for her to “understand that everyone is alone, not in the sense of loneliness, but in the sense that no-one can completely understand someone else.” Dijkstra also writes: “I want to awaken definite sympathies for the person I have photographed”. Dijkstra seems to wish to stress the solitude of human beings by removing any social interaction in her images, while at the same time, revealing the human essence of the sitter through a magnified representation of their physical characteristics. However, it is in their pose (which is spontaneously adopted by the models) that we might find reasons to feel disturbed by these images. These young people seem to be performing some kind of unsuccessful “fashion scene”; they project an idealised identity with their pose, and by doing so, they obscure their “true self”. Dijkstra has said that her interest in the beach series “was in the varied self-expression of youths in Poland and the United States, the latter unsparingly being more self – and fashion- conscious than the former”.
From Dijkstra’s words, we might understand that she aims through her work to demonstrate various sociological aspects concerning the young that could also be applicable to the rest of society. She seems to be concerned with the solitude of human beings in relation to their incapacity to interact with “the other”. In addition, she also proposes that part of this solitude might be produced by the fact that we are all surrounded by a world of commercialized images; a world that reduces our potential to understand our true identity and pushes young people to spontaneously perform idealized self-representations in front of the camera.
The ambiguity in the poses of these young people, together with the size of the image and the incredible amount of detail achieved by using a view camera, might provide the aesthetic reasons why this work has captivated a large public. But in terms of its conceptual reasoning, according to Stallabrass, the plausibility of these photographs lies in their ability to “describe and also enact a world in which people are socially atomized, politically weak, and are governed by their place in the image world”.
From the above discussion, it can be seen that identification plays an important role in the construction and reading of the photographic portrait, especially of the twentieth century. Depending on how the subject is presented, certain emotional responses, based on recognition or rejection are provoked in the viewer. In this way, through a relationship with a photographic human subject, there is the potential that the viewer, consciously or unconsciously, is led into contact with a part of themselves, which, in itself may be rejected or accepted. Eduard Steichen said, “The mission of photography is to teach man to man and each man to himself” If this statement can be taken as certain, then portrait photography might be the best way to fulfill such mission.
Contemporary” Portraiture: Facing the Subject, ed. Joanna Woodwall. 1997 Manchester University
Press, p. 239
Michael Jennings et al., trans Edmund Jephcott (Cambridge Mass.: Harvard University Press) p 520 as
quoted by Julian Stallabrass. What’s in a Face? Blackness and Significance in Contemporary Art
Photography, October Magazine, Fall 2007, Massachusetts Institute of Technology, p. 83
October Magazine, Fall 2007, Massachusetts Institute of Technology, p. 75
2003) as quoted by Stallabrass in What’s in a Face p. 78
1975-2001, Cambridge, Mass: MIT Press, 2004) p. 221, as quoted by Stallabrass in What’s in a Face,
Artforum 36, nº 9, (May 1997), as quoted as quoted by Stallabrass in What’s in a Face, p. 86