Because it’s evolutionarily unfavourable, there is a common assumption that animals will avoid mating with relatives. Inbreeding can lead to ‘inbreeding depression’: a reduction in available traits for offspring, making the population less genetically diverse and thus less able to adapt to their environments.
The paper examined 139 studies across 88 species, finding that animals rarely avoided mating with relatives. While some studies in the analysis found animals avoided inbreeding and others found they preferred it, on average there was no preference either way.
“We address the ‘elephant in the room’ of inbreeding avoidance studies by overturning the widespread assumption that animals will avoid inbreeding whenever possible,” says Raïssa de Boer, a researcher in zoology at Stockholm University in Sweden and first author on the paper.
“People assume that animals should avoid mating with a relative when given the chance,” she adds. “But evolutionary theory has been telling us that animals should tolerate, or even prefer, mating with relatives under a broad range of conditions for more than four decades.”
This rule also holds true for humans, showing people are not hardwired to avoid mating with relatives – something royal historians have been pointing out for centuries.
“We compared studies that asked if humans avoid inbreeding when presented with pictures of faces that were digitally manipulated to make the faces look either more or less related to studies that used similar approaches in other animals,” says de Boer. “Just like other animals, it turns out that there is no evidence that humans prefer to avoid inbreeding.”
The study also highlights publication bias in the field, suggesting that studies were more likely to get published if they showed evidence of inbreeding avoidance. There were more inbreeding avoidance studies published with smaller sample sizes and higher uncertainty. Larger studies, on the other hand, didn’t skew in either direction.
John Fitzpatrick, also a zoologist at Stockholm University and senior author on the study, says the findings have implications for conservation management.
“A primary goal of conservation efforts is to maintain genetic diversity, and mate choice is generally expected to achieve this goal. Our findings urge caution in the application of mate choice in conservation programs,” says Fitzpatrick.
Ellen Phiddian is a science journalist at The Royal Institution of Australia.
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