Their question is part of a model used by experimental economists to explain social and individual behavior.

VOA: special.2009.03.03

It says an explosion or explosions by comets could explain the disappearance of many animals and human beings who lived in North America at that time.

VOA: special.2009.01.20

The report says the changes could explain why persons born by Caesarean section are more likely to get diseases like diabetes and asthma later in life.

VOA: special.2009.11.24