In this lecture, we'll focus on the notion of population displacement, as a key social determinant of health in adolescents. Population displacement can occur as a result of natural disasters, such as typhoons, earthquakes, and tsunamis, with very significant population movements. It can occur from manmade disasters, such as war, and from other forces, such as immigration, whether as refugees, or as rural populations move to more urban regions, either within the same countries or, indeed, to different countries. Adolescents normally benefit from the influence of adult role models, social norms, and structures, community groups, the notion of social scaffolding in their lives. But when people are displaced, family and social structures are disrupted. Adolescents may be separated from their families or communities, while formal and informal educational programs are discontinued, and community and social networks break down. Adolescents may be scared, stressed, bored or idle. They may find themselves in risky situations that they are not prepared for dealing with, and they may suddenly have to take on adult roles without preparation, support, or positive adult role models. Adolescents who live through crises may not be able to visualise a positive outlook for themselves, and may develop fatalistic views about the future. The loss of livelihood, security, and protection provided by the family and community, places such adolescents at risk of poverty, of violence, and sexual exploitation, and abuse. While rural to urban migration is typically associated with perceptions of greater opportunities for education and employment for the young, there are also risks to health. These risks can result from the break down of the capacity of families and communities, to provide a safe and supportive environment for young people. Or, due to greater risks that result from large numbers of people living at close quarters, such as road traffic accidents, interpersonal violence, social alienation, stress, and the impact of thwarted expectations, especially around employment or relationships, can contribute to increased mental health concerns, sexual health risks, and substance use. It's interesting to think about what proportion of the world's population is living in large cities, including megacities, cities that have a population of greater than ten million people. Now, in about 1950, it was thought that about 30% of the world's population lived in cities. That increased to about 50% in the mid-2000s, and it's estimated that by 2015, about 60% of the world's population will be living in cities. And similarly if we go back to the 1950s, where there was one mega city and I'd ask you to think about which city you might think that might have been, about 10 years ago it was thought that there were about 23 megacities, populations greater than 10 million. And in the next few years, it's estimated that there will be 36 megacities in the world today, with over two-thirds of them in Asia. Interestingly, in the next few years, it's also estimated that nearly 60% of the population that lives in cities, is living in mega cities. Given the proportion of the population, thought to be between about 30 to 50% of mega cities, who live in the equivalent of urban slums, this is particularly concerning in terms of their health. Whether that's in terms of lack of security of clean water, of regular food, of safe housing, or indeed, greater exposure to a range of physical, emotional and sexual and reproductive health risks. In terms of population displacement, an important consideration is women's physical safety, which is not only a basic human right, but highly relevant to their sexual and reproductive health. According to the UN global agency, UN AIDS, up to seven in every ten women globally suffer violence from men in their lifetime. It is much harder and often impossible for women to negotiate for safer sex, for example, if they live in an environment of violence or fear of violence, which is more common in the context of population displacement. The executive director of another UN agency, UNFPA, Babatunda Osotimehin said, we will not be able to stop HIV, and improve women’s and girls’ health, until we empower women, advance gender equality, and engage men and boys in this effort. Intimate partner violence is associated with heterosexual transmission of HIV to women. For example, married Indian women, who are physically and sexually abused by their husbands, have a four fold increased risk of HIV infection, compared to women who are not abused by their husbands. And in a number of parts of the world, including India, despite reductions recently in the prevalence of HIV infection among the general population, women account for a rising percentage of all HIV cases, with husband's risk behaviour described as the major source of women's infection. Displaced adolescents are at greater risk of sexual exploitation and abuse. Because of their limited life experience, they may not recognise the sexual nature of abusive or exploitative actions in strange settings. Adolescents separated from their families, and adolescent heads of household, lack the livelihood security and protection afforded by the family structure, and are therefore more at risk from poverty, as well as from sexual exploitation and abuse. Separated adolescents, and adolescent heads of household, may also be compelled to drop out of school, marry, or sell sex in order to meet their basic needs for food, shelter, or, indeed, protection. Adolescent girls selling sex are then at risk of unwanted pregnancy, unsafe abortion, greater risk of STIs and HIV, as well as sexual exploitation and abuse. And survivors of sexual and gender based violence are similarly at risk of unwanted pregnancy, unsafe abortion, STIs including HIV, as well as mental health, psychosocial problems, and social stigmatisation. An example in post-earthquake Haiti is that a significant proportion of survivors of sexual violence treated by various non-government organisations, such as Médecins du Monde, were adolescent and even pre-pubescent girls. And we shouldn't forget that adolescents who are associated with the armed forces or other armed militia groups because in these environments, both boys and girls are often sexually active at a much earlier age, and sadly face increased risk of sexual violence and abuse, mental health issues including post traumatic stress disorder, and in the same range of sexual and reproductive health complications such as unwanted pregnancy, unsafe abortion, STIs, and HIV infection.