net positive or negative. If a country can increase its workforce productivity faster than its population is declining, the results, in terms of its economy, the quality of life of its citizens, and the environment, can be net positive. If it cannot increase workforce productivity faster than its population's decline, the results can be negative. National efforts to confront a declining population to date have been focused on the possible negative economic consequences and have been centered on increasing the size and productivity of the workforce. Causes A reduction over time in a region's population can be caused by sudden adverse events such as outbursts of infectious disease, famine, and war or by long-term trends, for example, sub-replacement fertility, persistently low birth rates, high mortality rates, and continued emigration. Short-term population shocks Historical episodes of short-term human population decline have been common and have been caused by several factors. High death rates caused by disease, for example, the Black Death that devastated Europe in the 14th and 17th centuries, the arrival of Old World diseases in the Americas during European colonization, and the Spanish flu pandemic after World War I, Famine, for example, the Great Irish Famine of the 19th century, and the Great Chinese Famine caused by the Great Leap Forward which caused tens of millions of deaths War, for example, the Mongol invasion of Europe in the 13th century, which may have reduced the population of Hungary by 20–40%, Civil unrest, for example, the forced migration of the population of Syria because of the Syrian Civil War Less frequently, short-term population declines are caused by genocide or mass execution. For example, it has been estimated that the Armenian genocide caused 1.5 million deaths, the Jewish Holocaust about 6 million, and, in the 1970s, the population of Cambodia declined because of wide-scale executions by the Khmer Rouge. In modern times, the AIDS pandemic and the COVID-19 pandemic have caused short-term drops in fertility and significant excess mortality in a number of countries. Some population declines result from indeterminate causes, for example, the Bronze Age Collapse, which has been described as the worst disaster in ancient history. Long-term historic trends in world population growth In spite of these short-term population shocks, world population has continued to grow. From around 10,000 BC to the beginning of the Early modern period (generally 1500 – 1800), world population grew very slowly, around 0.04%