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Sex Ratio: Definition, Calculations & Interpretations

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  • 0:01 Sex Ratio
  • 0:37 Various Sex Ratios
  • 3:10 Theories About Sex Ratios
  • 4:56 Lesson Summary
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Lesson Transcript
Instructor: Christopher Muscato

Chris has a master's degree in history and teaches at the University of Northern Colorado.

The sex ratio is a pretty important part of evolutionary life. Explore what this ratio is and what it means, then test your understanding with a brief quiz.

Sex Ratio

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?

Various Sex Ratios

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.

Theories About Sex Ratios

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.

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