In this section, we review population and fertility trends, factors that govern fertility, and the consequences of population trend.
Today's world population is approximately 7.7 billion people and growing to a projected 9.6-10.9 billion in 2100 . Uncertainty in the distant future depends on how fertility trends evolve.
Recent trends suggest, however, that the UN may be underestimating future population growth in Africa .
Unforeseen social trends could significantly alter population forecasts . The advent of life extension  and advancement in assistive reproductive technology such as in vitro fertilization  might modestly increase forecasts in the 21st century. It is also unclear the extent to which the current low fertility regime will induce natural selection pressures toward large families, causing an eventual rebound .
The main variable governing long-term population trends is the total fertility rate: the number of children born to women on average. TFR tends to be higher in lower income countries. Sustained over a long time, a TFR above about 2.1 and no net immigration means that a country's population will grow, and a TFR below 2.1 means that population will shrink .
Since 1964, world TFR has been decreasing.
Broadly speaking, there are three classes of explanations for falling fertility. The cultural evolution hypothesis that modernity tends to spread norms that encourage fewer births . The quality/quantity tradeoff holds that, as children's educational needs grow, parents make a rational decision to have few children while improving the education (quality) of those they do have . Modernity may reduce the impulse to have children by allowing the pursuit of competing desires, such as noncommittal romance or women's career fulfillment .
The following correlates with fertility rates have been observed.
It is debated how many births were prevented by China's One-Child Policy, the most notable coercive population control policy in history, though the policy is responsible for severe human rights abuses and long term social damage .
Economists generally argue that, in industrialized countries but not necessarily in poorer countries, a higher population should lead to higher per capita economic growth due to there being more researchers, more opportunity for specialization, and economies of scale , though see Coleman and Rowthorn  for an alternate view.
If coupled with investments in human capital, especially education, a transition from a high birth and death rate to a lower birth and death rate can free up resources from childcare and catalyze rapid economic development . Following a protracted period of low birth rates, countries experience an increase in the share of the retired population, which depresses growth .
Our analysis of the role of population growth in human environmental impacts is in progress.
One's views on population policy may be influenced by choice of ethical framework. Among utilitarian frameworks, an average utility model, as articulated by Hardin , holds that the average well-being among people is the morally relevant quantity. The total utility model holds the sum of well-being over all people to be the morally relevant quantity . A total utility view would generally be more pronatalist than an average utility view.
Relatedly, there is the question of how ethically to value a hypothetical future life brought into existence, relative to lives that already or will exist .
Philosophers do not have a concensus on the best way to resolve these issues.
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