Early studies on global cities focused on major metropolitan areas such as London, New York City, and Tokyo. However, research on growing global cities outside of this trio, such as Amsterdam, Frankfurt, Houston, Los Angeles, Mexico City, Paris, Sao Paulo, Sydney, and Zurich, has been performed throughout time. Today, there are more than 200 global cities with populations over 1 million people. They account for about 70% of the world's population and 75% of its economic activity. Global cities are defined by their interdependence on trade and investment between countries. Thus, they share many common characteristics that set them apart from other large cities. For example, they tend to have dense central business districts with a high ratio of office space to housing, well-connected public transport systems, and extensive networks of parks and green spaces. They may also have cultural attractions such as museums, universities, and libraries that help attract tourists and businesses, respectively. Some authors have argued that global cities are becoming increasingly similar to one another because they all face the same challenges regarding urbanization, climate change, poverty, and violence against women. However, they do differ in terms of their histories, cultures, and economies, which helps explain why some grow rapidly while others decline.