In the 1970s and 1980s in Japan, for the first time, several studies identified symptoms of “withdrawal neurosis” [2,11] in situations where patients decided to withdraw from their everyday contexts, such as school or work. Early cases of withdrawal (occurring mostly among young people) were often confused with various existing psychopathologies, such as schizophrenia or depression. Due to the increasing attention paid to these symptoms by various scholars, more in-depth studies of the disorder began to be conducted, eventually recognizing it as a real syndrome different from all others already known [5,11,12].

In the 1990s, these symptoms were traced to the syndrome that is now known and identified as Hikikomori [2,3,5,13,14]. Since voluntary isolation is also a prodromal symptom of schizophrenia, in early diagnoses, Hikikomori syndrome was treated with the drug and nursing therapies provided for schizophrenic patients [2,5]. However, in Hikikomori, all of the symptomatology that greatly alters the perception of reality present in schizophrenics is absent. Far from being considered persons incapable of understanding, those who become Hikikomori first and foremost choose it; that is, they decide it with lucidity [15,16].

According to some authors [11,13,16,17], Hikikomori can be classified as a true state of anomie, a term coined by Durkheim [18] that means “absence of norms”. However, their loss of normative references is not related to the general context, but only to those aspects related to social dynamics, personal success and one’s public image. It would thus be a form of narrow anomie, as other norms, such as moral ones, are maintained.

One of the first general characteristics that emerge from early studies on Hikikomori syndrome is the slow course of the pathology, which does not manifest itself suddenly and blindly, but as a long process. Crepaldi [19] tried to identify the stages that a Hikikomori goes through before total isolation. What can be inferred is that the isolation resulting from Hikikomori syndrome is not a choice dictated by a bad personal moment. It is a reasoned choice that the person matures over time and in full clarity. In the first stage, the person begins to feel the urge to isolate himself or herself, finds relief in isolation and feels uncomfortable with other people; in the middle stage, the person begins to consciously process the impulse to isolate, quits school or work, reverses the sleep-wake cycle [3,20] and begins to spend most of their time in their own home [19].

Among the main reasons why young people become Hikikomori is the pressure for social achievement and the race for perfectionism [2,5]. In young people, the onset of Hikikomori disorder often coincides with the end of school age (age 19) and the beginning of adulthood, when important decisions about one’s future must be made. At this delicate stage, those who have been termed “emerging adults” [16,21], ages 18–29, must build their future and identity. In high-income societies, young people are often under constant stress caused by the competitive climate at both school and work. In this regard, other reasons related to the sociocultural context of young people in self-isolation have been highlighted: frequent incidents of bullying suffered and bad relationships with peers [13,22]; the pressure exerted by schools and inattention of educational institutions toward young people who are Hikikomori or prone to become Hikikomori [4,11,23]; and an overly protective and comforting family context [2,4,5,23].

Japan is considered the home of the Hikikomori phenomenon [24,25], and to date it remains the country with the largest number of isolated children. Data dating back to 2011, reported in a study by Takasu et al. [20], showed that about 200,000 people in Japan can be classified as Hikikomori (out of a population of about 120 million). One of the reasons that could be attributed to the phenomenon is certain Japanese cultural traits. From a cultural point of view, in Japan great importance is placed on “sekentei” (世間体), which means “appearance in the eyes of others” and reflects social expectations that the individual cannot disappoint. Therefore, dependence and conformity in Japanese culture are part of cultural values that can guide and condition each individual’s being in society. According to several authors [4,6,16,23,24,26], the boys who are most likely to become Hikikomori are those who prefer to spend a lot of time alone, have an economically well-off family, perform well in school, and are often bullied. Along with these “personal” factors, there are some contextual factors that facilitate withdrawal; school and/or university pressure, fear of facing new realities (many Hikikomori decide to isolate themselves at the end of a school career, ready to start a career path), and finally Japanese work environment, which is very competitive.

In 2014, Fansten et al. [27] hypothesized the existence of different blocking modes in the socialization process of Hikikomori. Their study proposed the construction of a typology to define the different types of Hikikomori, later revised by Crepaldi [19]. Currently, from the typological classifications developed by these scholars, four types of Hikikomori can be distinguished:

• Alternative: Those who belong to this type decide to isolate themselves because they do not accept the social dynamics of modern society. Individuals categorized in this way isolate themselves to avoid “regular” adolescence and rebel against society. Is possible that in this case, the boy before reclusion is preceded by a severe existential depression;
• Reactionary: Those in this group experience an many times familiar, context, already difficult one that has contributed to exacerbating an already pre-existing. Imprisonment is a reaction to their already difficult situation and is linked to an especially traumatic events;
• Resigning: This type of withdrawal concerns those individuals who cannot withstand the pressure arising from others’ expectations. The subjects manage to alleviate their malaise by isolating themselves by abandoning social competition, whether in school and work settings;
• Chrysalis: The person in this typology seeks an escape from the responsibilities of future adult life in isolation. He feels that he does not have the skills to deal with the future, which is seen as a source of anxiety; therefore, every thought about it is avoided through the mechanism of avoidance. The Hikikomori is like freezing time and adopting strategies, unconsciously or consciously, aimed at flattening one’s life and trying to freeze the present. There is an alteration in the mechanisms of nourishment, eating meals in a way that is fast and irregularly.

On the other hand, Li and Wong [26] proposed a classification of the Hikikomori syndrome about whether or not the person is affected by other diseases. In this regard, the distinction between primary Hikikomori and secondary Hikikomori has been proposed. In the former case, the syndrome is not related to any pre-existing psychopathology of the person. Secondary Hikikomori, on the other hand, is when the confinement is a direct consequence of an already present problem, such as anxiety disorders or obsessive-compulsive disorders.

Over the past decade, studies on Hikikomori syndrome have spread outside Japan and around the world, allowing for a deeper understanding of a condition that is as silent as it is extensive. Major studies include those conducted in France [27]; in Italy [19]; Canada [28]; Australia and South Korea [29]; Spain [30]; Belgium [31];

Although studies have spread to most states in industrialized society, there is still a gap that has not been closed. Although Hikikomori disorder was initially treated with depression and anxiety, this is fortunately no longer the case today. However, there is still a lack of clear diagnosis to identify ad hoc treatment for this type of patient [13].

Some recent studies have pointed out the strong relationship between Hikikomori and the use of technological devices, especially the Internet. The development and spread of the Internet have influenced the behavior of people living ordinary lives and that of Hikikomori. While early case studies showed no link between Hikikomori and technology use habits [5], more recent studies [26,29,32,33] show the opposite: Hikikomori spend most of their day online, mostly playing online games, developing in many cases what is now recognized as Internet Gaming Disorder (IGD) and Internet Addiction (IA). The relationship between these disorders and the pathology of Hikikomori continues to intensify, as evidenced by studies such as that of Tateno et al. [34], which show an increase in the amount of time spent online. In recent years, the intensified coexistence of these disorders has led to confusion, resulting in the overlap between them, to the point that the pathology of Hikikomori is confused with both Internet Gaming Disorder and Internet Addiction, and conversely [29,33,35]. For Hikikomori, the Internet is an escape route since using it they can create new relationships and use that Web-mediated sociality as a substitute for the sociality lost by isolating themselves at home [34,36,37].