Until recent decades, the transition to farming was seen as an inherently progressive one: people learnt that planting seeds caused crops to grow, and this new improved food source led to larger populations, sedentary farm and town life, more leisure time and so to specialization, writing, technological advances and civilization. It is now clear that agriculture was adopted despite certain disadvantages of that lifestyle. Archeological studies show that health deteriorated in populations that adopted cereal agriculture, returning to pre-agricultural levels only in modern times. This is in part attributable to the spread of infection in crowded cities, but is largely due to a decline in dietary quality that accompanied intensive cereal farming. People in many parts of the world remained hunter-gatherers until quite recently; though they were quite aware of the existence and methods of agriculture, they declined to undertake it. Many explanations have been offered, usually centered around a particular factor that forced the adoption of agriculture, such as environmental or population pressure.