Inbreeding is Depressing

Sex-specific inbreeding depression: A meta-analysis (2021) Vega-Trejo et al., Ecology Letters,

Image credit: Monica R., CC BY 2.0, Image Cropped

The Crux

One very basic rule in nature is that it is bad to produce offspring with a close relative. The loss of fitness associated with this sort of breeding is called the inbreeding depression, and it happens because inbreeding leads to a greater chance that recessive or deleterious (i.e., bad) alleles will be expressed. Though inbreeding affects both male and female offspring, it is unknown as to whether or not there is a general rule of it affecting one sex more than another. Today’s authors sought to answer that question by using one of my favorite statistical techniques: the meta-analysis (see Did You Know?).

Did You Know: Meta-analysis

A meta-analysis is an analysis of other, previous analyses. A study of studies, if you will. What a meta-analysis entails is searching the scientific literature for studies pertaining to a specific question, collating all of those results, and then running statistical models on those collective results. There are many different techniques and methods used to control for various differences among the different studies, but the long and short of it is that meta-analyses allow for generalizable results and statements relating to a given question.

What They Did

The authors searched the scientific literature for experimental studies that estimated inbreeding depression in males and females. These studies had to include an experimental comparison between inbred and outbred offspring, or those whose parents were close relatives and those whose parents were not closely related. These studies had to look into both male and female offspring, to allow a direct comparison between the sexes. For example, an effect of inbreeding on female body mass is directly comparable to same effect on male body mass.

From the 2,685 studies they found, the criteria above meant that only 79 were eligible for inclusion in the meta-analysis. From these studies, the authors calculated 321 effect sizes, which are representations of the magnitude of the effect in question. They used Hedge’s g, which is also known as the standardized means difference. This effect size represents the difference between the mean values of two groups (e.g., inbred and outbred), and is a good way to understand if one group is “better” than the other in terms of fitness.

What They Found

In general, outbred individuals are moderately more fit than inbred individuals, though there was a large amount of variation among the effect sizes. Interestingly, though both male and female offspring suffered from inbreeding depression, the effect was stronger in females than males.

Figure 2 from today’s paper. On the left is a phylogenetic tree showing the various species included in this study, and the numbers on the right show the number of studies/effect sizes from a given species. Though this is only part of the figure, the key takeaway is that most of studies/effect sizes relate to arthropods, which make up the upper of the two main branches (Image credit: Vega-Trejo et al. 2021, Ecology Letters)


The authors found evidence for a publication bias in the studies they included in their analyses. A publication bias means that the papers published tended to be biased towards a given result, and in the case of this study that means that the papers they found were biased towards a “positive” result, or a result showing an effect of inbreeding depression. This is a common issue in meta-analyses, as papers with “positive results” are published more than those with “negative results”. Additionally, the studies included were biased towards arthropods. The methods the authors used thankfully accounted for this publication bias, and the data is what the data is so they can’t help the bias towards arthropods. But, it makes me wonder if and how their results would have changed if these biases weren’t present.

So What?

Many patterns and phenomena in the natural world differ among the sexes, and this study showed evidence for yet another sex-specific difference. Females suffer more from inbreeding than males, but there is still much left to understand related to this question. For example, though the mechanisms driving this pattern weren’t investigated here, they are in fact likely to depend on the species in question. Hopefully these results inspire a plethora of future studies to understand just that.

Dr. Adam Hasik is an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions who grew up in Alabama and has heard every inbred joke you could imagine. You can read more about his research and his work for Ecology for the Masses here, see his personal website here, or follow him on Twitter here.

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