About the Authors
Population Division, Department of Economic and Social Affairs, United Nations, New York, New York, United States of America
The author has declared that no competing interests exist.
Conceived and designed the experiments: CCS. Performed the experiments: CCS. Analyzed the data: CCS. Wrote the first draft of the manuscript: CCS. Contributed to the writing of the manuscript: CCS. ICMJE criteria for authorship read and met: CCS. Agree with manuscript results and conclusions: CCS.