1. The social exclusion
Human beings need social relations. The evolutionary point of view suggests how belonging to the group is a fundamental aspect in social life due to survival. The breakdown of social interactions predisposes to health risk factors even worse when the break occurs because they are intentionally excluded from other people. This experience can cause an experience of severe psychological distress, in which strong negative emotions of sadness and depression, a high level of stress was expired. Why does it happen? Group membership for people is a fundamental requirement for safety, reproductive success, mental health, and physical well-being [1, 2, 3, 4]. The experience of social exclusion breaks all this.
Being excluded is a very stressful experience that produces negative emotions and thoughts, which if prolonged over time can also cause a weakening of the immune system and lead to physical illnesses. The concept of group belonging is closely associated with one’s self-esteem.
Tajfel and Turner , two important authors for their studies on identity, indicate how group membership defines social identity. Social identity is the image of oneself that emerges from the awareness of belonging to one or more social groups, characterized by their value and characteristics. But what is social exclusion?
Eisenberger  defines social exclusion as one of the most painful experiences that human beings can live. Williams  argues that typically the term “excluded” is also defined as “being ignored,” in a situation in which a group marginalizes an individual. In fact, he explains how the experience of social exclusion can be experienced also when an individual is ignored by an only person or worst condition when he is ignored by a group. Duration of social exclusion can modulate the quality of life. According to this aspect, Riva et al. , in their research, investigate how the short or prolonged exposition of social exclusion can affect health. Their results indicate that a short duration allows a more rapid recovery of psychological needs; instead when it is prolonged, it becomes chronic. In detail, authors suggest the possibility that chronic exclusion and chronic pain cause common psychological responses, with higher levels of negative emotions and worse prognosis.
The comparative use of the words physical pain and social pain is supported by neuroimaging researches. In fact, several studies [9, 10, 11] demonstrate the involvement of the brain areas of physical pain during the experience of social exclusion.
2. The effect of exclusion
Social exclusion is usually associated with the risk of poverty and marginalization. The social problems connected to exclusion are very important from various points of view, from the sociological to the economic one of any type of society.
This kind of experience is usually associated with social situations and social categories at risk, for example, people who lose their jobs, single women, immigrants, disabled, and homeless. In reality the experience of social exclusion can be experienced painfully by each of us when we are ignored or excluded from the people/groups to which we belong.
There are periods in life where people are more vulnerable and sensitive to this kind of experience, for example, the adolescence. Adolescence is the most important period for social relations with peers, during which the need for group membership becomes the most important aspect of life . Two factors that greatly affect this stage with peers are acceptance and popularity [13, 14]. Exclusion derived by peer can create a rupture of acceptance and create a negative and intense painful experience, often evoked by bullying.
Bullying is a widespread phenomenon in many countries; it is characterized by violent physical and verbal behavior but also indirect aggressions such as the psychological one. The bully acts these behaviors on the victim intentionally and repeatedly over time. Bullying behaviors can be more difficult to detect when they are done through social exclusion strategies from the group to which they belong .
Today there are also risks associated with new technologies, with the emergence of new forms of social exclusion, such as cyberbullying and sexting. In the first case, many adolescents are insulted, mocked, and attacked on social networks so much that they have to change schools; another example of exclusion and social violence is sexting, which is sending of sexual messages via electronic devices.
This phenomenon represents risk factor for suicidality in adolescence; adolescent suicide is a very serious problem that affects all countries of the world. Koyanagi et al.  have defined it as a global public health issue. In this regard, authors  report the need to implement interventions to address these issues, associated with the frailty of adolescents so as to prevent suicide and suicide attempts among adolescents around the world.
The topic of suicide is very difficult to deal with because it involves many aspects related to the culture of various countries, such as religion, cultural traditions, etc. However, there are some realities in the world that have managed to overcome the stigma of the meaning of suicide by promoting school prevention programs. An important example, considering the string of religious tradition of Italian culture, is the La Tazza Blu Italian association that deals with suicide prevention for adolescents (11–19 years of age) with a structured program of prevention and intervention.
In addition to this association, there are other very important ones in the world, but it is important to be able to overcome the cultural barriers that prevent dialog and confrontation on these issues that are so delicate and sometimes so difficult to face in some cultures.
It is important to remember that the fragility of the experience of social exclusion is also a typical condition in old age that can lead to social isolation. So the concept of social exclusion concerns and can affect everyone because it reflects the fragility of all people.
As reported by Brewer , the study of social exclusion concerns the investigation of a complex phenomenon that can be analyzed by different disciplines, from different points of view with possible boundaries of convergence and divergence. Through the application of different research methodologies, from the qualitative one of social psychology and sociology to the quantitative one of neuroscience, it is possible to deepen this theme in order to give a greater contribution to the understanding of the phenomenon. Being able to better understand the risk factors of vulnerability can help to put in place useful strategies of social inclusion . It is important to better understand this research topic because, as described in this introductory chapter, the consequences of experiences of social exclusion can also lead to suicide and therefore to death.