Back To CourseSociology 104: World Population
8 chapters | 88 lessons
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Chris has a master's degree in history and teaches at the University of Northern Colorado.
Boy meets girl. It's a tale as old of time, right? Generally, we expect this classic love story to go a certain way because we expect most societies to look like this. But, what if the world looked like this? Or like this? Suddenly that love story becomes a bit different, doesn't it? The ratio of males to females in a population is called the sex ratio, and it can have a pretty big impact. See what I mean?
The sex ratio, at its broadest, applies to any species with male and female sexes. But when we use the term sex ratio, we actually could be talking about a few different things. The primary sex ratio is the ratio at fertilization, or the sex ratio of unborn offspring in a population. The secondary ratio recalculates this as the ratio at birth, and the tertiary sex ratio is the ratio at sexual adulthood. There's actually a fourth one as well, the quaternary sex ratio, which calculates the male/female ratio in adults past the age of sexual reproduction. This last one can be a bit problematic since that age varies greatly by sex. But, by measuring these four different ratios, you get a good idea of what's going on in a population throughout an average lifetime. Ideally, all of these ratios should stay roughly balanced, but imagine that one of them, let's say the ratio at adulthood, is greatly off. That means that somewhere between birth and adulthood, you have a lot more females dying than males, or vice versa, so now you know there's an issue here that needs addressing.
For almost all species, including humans, the average sex ratio is about the same at 1:1 which means an average of one male to one female. That's a simple ratio, although it's also common to see it in terms of 100s. A perfectly balanced ratio in that form would look like this: 100:100. Like I said, most human societies have a roughly balanced sex ratio, with the global human sex ratio sitting at 107:100, so 107 males to every 100 females. That's close enough to still be simplified as a 1:1 ratio, but many scientists are puzzled by those extra seven males. Why do human populations tend to have a few more boys than girls? Well, most explanations of human sex ratios chalk that up to the fact that we are the only species that really has any measurable control over reproduction. Many societies across history have had strong preferences about male or female children, as well as gender roles in terms of war and labor, and these preferences can have a major impact on which children are more likely to survive all the way from conception to adulthood.
The sex ratio, as it naturally appears without too much human alteration, is generally assumed to be the natural balance that will best ensure a species' survival. This was established back in 1930, by English statistician and biologist Ronald Fisher. According to Fisher, a ratio of 1:1 is the most stable from an evolutionary perspective, an assumption since referred to as Fisher's principle. So, a population with a roughly 1:1 sex ratio is often referred to as Fisherian, and modern researchers still agree that this is the hallmark of an evolutionarily stable population.
But, remember how human populations tend to have seven more males than females per hundred? Actually, it's pretty common amongst most species for the ratio to be ever-so-slightly tipped towards males. But why? Basically, guys are expendable. . . sorry, dudes. But historically, men have had higher death rates than females. Part of this is a result of warfare but also diseases that are more common to men, such as heart attacks. So, in order to maintain a roughly balanced population, evolution compensates for the higher rates of death amongst males by adding a few more guys.
However, that's still just one theory about sex ratios. There are many others, and researchers disagree about exactly how a population is able to maintain a balanced sex ratio. But from what we can tell, this sex ratio has been generally consistent for millennia, especially across human history. It's a classic story, a tale as old as time.
The sex ratio is the ratio of males to females in a population. A balanced ratio is 1:1, or an average of one male for every one female. This can also be expanded to be read in terms of hundreds, so 100:100. That's a perfectly stable sex ratio, although the global ratio for humans is around 107:100. That's still pretty stable.
Most researchers believe that a perfectly balanced sex ratio is the most stable from an evolutionary perspective, an assumption called Fisher's principle. However, human sex ratios do tend to lean slightly towards more males than females, which may be explained through the fact that the death rates are higher for males than females. So, evolution compensates by slightly favoring male offspring.
Sex ratio can be measured at conception, at birth, at sexual adulthood, and at post-reproductive adulthood, and each of these reveals important trends in life expectancies. But, if your sex ratio remains balanced throughout, then you've got a much better chance of boy meeting girl. Classic love story.
|Sex ratio||the ratio of males to females in a population|
|Primary sex ratio||the ratio at fertilization|
|Secondary ratio||recalculates this as the ratio at birth|
|Tertiary sex ratio||the ratio at sexual adulthood|
|Quaternary sex ratio||calculates the male/female ratio in adults past the age of sexual reproduction|
|1:1||means an average of one male to one female|
|100:100||a perfectly balanced ratio|
|107:100||the present global human sex ratio meaning 107 males to every 100 females|
|Ronald Fisher||English statistician and biologist who established back in 1930 that there was a prefect sex ratio|
|Fisher's principle||states the ratio of 1:1 is the most stable from an evolutionary perspective|
|Fisherian||modern researchers still agree that this (almost 1:1 ration) is the hallmark of an evolutionarily stable population|
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Back To CourseSociology 104: World Population
8 chapters | 88 lessons