Will Africa have most of the world's largest cities in 2100?

This paper responds to the article by Daniel Hoornweg and Kevin Pope, on predictions for the world’s largest cities in the 21st century, in this issue of Environment and Urbanization. It recognizes the value and importance of this article in highlighting the very large likely scale of urban population growth up to 2100 and in initiating a discussion on what this might imply for the scale and distribution of the world’s largest cities. But it raises some concerns about the extent to which very large cities will grow in what are currently nations with very low per capita incomes. Mega-cities need to be underpinned by mega-economies. The world’s largest cities up to 2100 will mostly be those where private capital has chosen to invest, and much of this may not be in the cities identified in the Hoornweg and Pope article as likely to be the largest. The economic future, the development future (including whether the Sustainable Development Goals get met) and the ecological future (especially whether dangerous climate change is avoided) will so powerfully influence future city sizes

Beyond Stereotypes of Adolescent Risk Taking: Placing the Adolescent Brain in Developmental Context

Recent neuroscience models of adolescent brain development attribute the morbidity and mortality of this period to structural and functional imbalances between more fully developed limbic regions that subserve reward and emotion as opposed to those that enable cognitive control. We challenge this interpretation of adolescent development by distinguishing risk-taking that peaks during adolescence (sensation seeking and impulsive action) from risk taking that declines monotonically from childhood to adulthood (impulsive choice and other decisions under known risk). Sensation seeking is primarily motivated by exploration of the environment under ambiguous risk contexts, while impulsive action, which is likely to be maladaptive, is more characteristic of a subset of youth with weak control over limbic motivation. Risk taking that declines monotonically from childhood to adulthood occurs primarily under conditions of known risks and reflects increases in executive function as well as aversion to risk based on increases in gist-based reasoning. We propose an alternative Lifespan Wisdom Model that highlights the importance of experience gained through exploration during adolescence. We propose, therefore, that brain models that recognize the adaptive roles that cognition and experience play during adolescence provide a more complete and helpful picture of this period of development

Internal versus external growth in industries with scale economies: A computational model of optimal merger policy

We study optimal merger policy in a dynamic model in which the presence of scale economies implies that firms can reduce costs through either internal investment in build- ing capital or through mergers. The model, which we solve computationally, allows firms to invest or propose mergers according to the relative profitability of these strategies. An antitrust authority is able to block mergers at some cost. We examine the optimal policy when the antitrust authority can commit to a policy rule and when it cannot commit, and consider both consumer value and aggregate value as possible objectives of the antitrust authority. We find that optimal policy can differ substantially from what would be best considering only welfare in the period the merger is proposed. We also find that the abil- ity to commit can lead to a significant welfare improvement. In general, antitrust policy can greatly affect firms` optimal investment behavior, and firms` investment behavior can in turn greatly affect the antitrust authority`s optimal policy