1With the article on the masculinization of births Population is launching a new series of annual chronicles coordinated by Dominique Tabutin and devoted to the state of world knowledge on a topical demographic issue. Targeting a broad audience of scientists, students, journalists and more, these chronicles will combine factual data with an overview of the theoretical and political aspects of the question in hand. By approaching topical issues in historical perspective, these articles will shed light on their wider context. After a critical description of information sources and measurement tools, an overview of recent research will describe ongoing trends and pinpoint social and spatial disparities. A discussion section will then explore the political or legal implications of the current and future situation, and the challenges facing researchers. Statistical and methodological appendices may also be included.

2The new series is inaugurated by Christophe Z. Guilmoto, who examines the question of the sex ratio at birth, which in humans naturally stands at around 105 boys per 100 girls. Scholars have long been interested in measuring variations in this ratio at different times in history, and among different populations or communities. The first accurate estimate was made by John Graunt in 1661, who found a ratio of 106.8 boys per 100 girls among infants christened in London. According to United Nations estimates, recent changes in sex ratios in certain populous countries of the world, such as India and China, have raised the global sex ratio from 105 in 1975-1979 to 108 in 2005-2009 (World Population Prospects: The 2012 Revision). These recent increases in the proportion of male births have been attributed to gender discrimination. In this article, the author analyses the mechanisms and causes of this phenomenon and discusses its social and political implications.