2. Was Malthus an optimist? (version 0.2)

Or, more accurately, the super-exponential nature of population growth.

According to Thomas Malthus population tends to increase exponentially. In fact population tends to increase super-exponentially. Even if economic, technological or other factors temporarily push fertility below replacement (as is the case in many first world countries), population will tend to increase super-exponentially in the long run.

A. Malthus’s model

Malthus's model assumes that fertility rates are fixed. In fact fertility rates are increasing in most first world countries and decreasing in most third world countries. Malthus’s error was assuming that a population is homogeneous and ignoring factors like technology.

B. A better model

Imagine that a population contains five groups.

Group    % of population    Annual growth rate
A        30                 -1%
B        30                 0%
C        20                 1%
D        10                 -2%
E        10                 2%

Assume that growth rates for each group are constant. The population is currently shrinking, but gradually the population growth rate will approach the growth rate of its fastest growing subgroup (2% per year). This model is an improvement over Malthus's model because it recognizes that populations are not homogeneous. A single high fertility group will elevate a population's growth rate in the long run. Israel is an example of a first world country with a high population growth rate because a large proportion of its population belong to a high fertility group (ultra-Orthodox Jews). By the end of this century many first world countries are likely to be in Israel's situation. But even this model is a simplification. It assumes subgroups are homogeneous and fixed and ignores factors like technology.

C. An even better model

Factors that affect fertility can be grouped into three categories: genes (alpha factors), beliefs or "memes" passed from parents to children (beta factors), and all other factors (gamma factors). Over time alpha and beta factors will become more and more pro-fertility. Unless gamma factors perpetually become more anti-fertility, the fertility rate will increase indefinitely leading to super-exponential growth. The fertility rate declined in the 20th century because gamma factors became more anti-fertility: contraception and certain economic factors (see "A Treatise on the Family" by Gary Becker) encouraged lower fertility, but this trend is unlikely to continue. In fact improved infertility treatment and increased legal incentives for parents mean gamma factors in first world countries are becoming more pro-fertility.

Even this model is somewhat of a simplification. In particular it ignores interactions between factors. Traits that were once pro-fertility like a high sex drive are less pro-fertility because of contraception. Nonetheless these interactions do not change the main results of this model: 1) pro-fertility alpha and beta factors will become more common and 2) the population will grow super-exponentially unless gamma factors perpetually become more anti-fertility. The model also ignores "mutation drag", but mutation drag will not be important until fertility rates become very high.

D. Consequences

Fertility rates in first world countries will increase greatly this century. Fertility rates in third world countries will decrease over the next several decades as gamma factors become more anti-fertility, but they too will begin to increase later this century. Characteristics associated with high fertility will become more common. This means conservative religious beliefs will become more common in first world countries despite factors like the internet which would otherwise erode conservative religious beliefs (see "Religion, fertility and genes" by Robert Rowthorn, who makes a slightly different but related argument).

A larger population is not necessarily a bad thing. It means more people can experience life, and it may mean faster innovation (see "The problem with transhumanism"). But a larger population, especially one that is very conservative and religious, will make it more difficult to confront the environmental challenges of the future.

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