Once, while I was teaching a class on genetics to dog trainers, my students asked me, “What percent of dog personality is due to genetics, and what percent is due to environment?” Unsatisfied with my answers of “it’s complicated” and “that isn’t the right question,” they persisted until I added a new lecture to the class tackling their question. I argued to them the question was actually nonsensical – personality isn’t something that can be quantified with numbers and percentages. I believe the real question they were asking was “Can genetics be absolute?” In other words, can a dog be dealt a bad hand genetically, with no chance of amelioration, doomed to bad behavior no matter what their environment? Real world versions of this question include “Are pit bull-type dogs genetically aggressive?” and “Is my dog aggressive because of poor socialization (my fault) or bad genetics (the breeder’s fault)?” Researchers haven’t yet found a way to conclusively answer these questions, but there is a body of work in the scientific literature that sheds some light on them.
The scientific term for the concept of “how much of a trait is inherited and how much is due to environment” is heritability. Heritability can’t be measured in an individual dog. How would you design such a study? Change a dog’s genetics and see if his personality changes? (Maybe in a few decades.) Go back in time to see if a different socialization approach with the puppy would cause differences in the adult? (Because when I say “the dog’s environment,” I don’t just mean the dog’s current environment, but his past environment as well, all the way back to conception.)
One approach is to look at large numbers of dogs and measure a particular trait in both parents and offspring. There is in fact an equation to measure the effects of heredity in that population of dogs:
Heritability = Variation due to Genetics / Variation due to Environment
Heritability is measured from 0-1; the variation in a trait with a heritability of zero is entirely due to environment, and the variation in a trait that measures one is entirely due to genetics. For example, whether a dog responds to a “sit” cue or not is almost entirely due to environment (a “zero” trait) – it depends on whether he was trained what that word means and how to respond to it. But a dog’s eye color is entirely due to genetics (a “one” trait). Weight is a great example of a trait that is due to both environment (how much the dog is exercised and fed) and genetics (how much the dog wants to eat, and how tall the dog is), and would have a number somewhere between zero and one. The heritability of weight hasn’t been measured in dogs so far as I know, but in humans it appears to be around 0.8 (80%).
It’s an understandable urge, then, to look at a trait with 0.8 (80%) heritability and conclude that (for example) “this individual’s weight is 20% due to environment and 80% due to genetics.” The most unintuitive part of heritability is that it applies only to populations, never to individuals. To unpack the heritability of weight, perhaps some dogs have a genetic background that tunes their metabolic rate high, so that they have a low weight no matter how much they eat. Other dogs might have a genetic background that tunes their metabolism to a more moderate rate, so that their diet and exercise regimen have a much greater effect on their weight. On average (measuring the effects of genetics versus environment in the entire population), weight would look like a moderately heritable trait: some influence of genetics, some of environment. When you look at individuals, however, some dogs will be roughly the same weight in most environments, while others will add or drop weight readily in response to changes in diet or exercise. In other words, genetics would have greater effect on some dogs, while environment would have a greater effect on others. That’s what I mean when I say that heritability is about populations, not individuals.
Behavioral traits, the same as with weight, fall in the middle between zero and one, reducing our question to “is the heritability of behavioral traits closer to zero or closer to one?” In theory, then, a behavioral trait with a smaller heritability (closer to zero) will be more affected by environment in general (even if the exact heritability number calculated in a study can’t tell us anything about the interplay of genetics and environment in an individual dog). A dog’s personality is made up of a number of smaller behavioral traits, but how to break personality down into individual behavioral traits is still a matter of much debate, as existing research on the heritability of canine behavioral traits shows.
Research on Heritability of Behavioral Traits in Dogs
Luckily for researchers who are interested in heritability of dog behavioral traits, there is a large data set to work from for some dog breeds. The Dog Mentality Assessment (DMA) is a standardized behavioral assessment that is structured much like a shelter behavioral assessment. In this test, a judge assesses the dog’s responses to a variety of stimuli, such as approaching strangers and loud noises. This information is used in Sweden to make breeding decisions for some populations of dogs, so the results are available for thousands of dogs. Pedigree information on these dogs is also available and the test has been performed on related dogs across multiple generations, so it’s perfect for heritability studies. This is a great research resource, all that information sitting in a database waiting to be used, and what studies using this resource strongly suggest to us is that behavioral traits fall between 0 and 1 – they are not determined solely by genetics or solely by environment, but by interactions between the two:
|Courage||0.28||Wilsson and Sundgren, 1997
(GSDs and Labrador retrievers)
|Ability to cooperate||0.28-0.35|
|Nerve stability||0.18||Ruefenacht et al., 2002
|Reaction to gunfire||0.23|
|Playfulness||0.26||Strandberg et al., 2005
Problems with Existing Dog Heritability Research
It isn’t straightforward to describe a dog’s personality. The DMA is generally administered by a professional who understands dog behavior well, but on the other hand, DMA test results seem to depend on who is actually administering the test, meaning that the results you get may have more to do with whether the tester is a harsh or lenient grader, rather than the dog’s actual personality. Also, a dog’s test results may also vary day to day – if the dog is feeling cranky that day, he may score differently than if he’s in a good mood. Therefore, I don’t really trust the results of these studies; I think the problems with describing a dog’s personality are still too great. What are we actually describing with the DMA – the dog, or the handler’s perception of the dog? Moreover, when we describe a dog’s “sharpness,” what do we actually mean by that, and how applicable is this trait to any given real-world situation?
What do We Actually Know about Heritability of Behavior in Dogs?
Most of these papers reported heritabilities of behavioral traits in the 0.1 – 0.3 range. I recommend taking these numbers with a grain of salt, as the DMA may not be a very reliably way of measuring personality. In reality, I think that the heritability of behavioral traits may actually be higher—if you can measure the trait well. Currently, I think we have no real idea how to measure behavioral traits. We can measure how a dog responds to a particular stimulus; a dog may be labeled “dog aggressive” if they are likely to attack other dogs. But is dog aggression a binary trait, in other words, can you simply classify every dog as either “aggressive” or “not aggressive” and be done? Clearly not, as some dogs are more aggressive than other dogs, and some are only aggressive in certain situations. How do you quantify it, though? By frequency of attacking other dogs? By the ferocity of the attack? By how difficult the dog is to call off the attack? More important than quantification of behavioral traits, though, is accurate labeling of them. Is dog aggression one trait, or many? Should we separate fear aggression from territorial aggression? Most importantly, we do not yet understand the biological differences in the brains of dogs with specific behavior problems from the brains of behaviorally healthy dogs. It is possible (and, the more we learn about behavior, very likely) that problems with one outward manifestation, such as dog-dog aggression, may have a variety of different biological mechanisms in different individuals. Treating all of these different cases as having a single cause for the purpose of a heritability study will result in a heritability measurement that doesn’t reflect reality.
In fact, I think the kinds of behavioral traits we should be studying, rather than “aggression” or fearfulness,” should be traits like “how high the dog’s stress hormones spike thirty minutes after a loud noise.” These sorts of low-level biological traits, I suspect, are much more heritable than the highly complex behaviors, like aggression, that we’d really like to understand. Those more complex behaviors, on the other hand, may not be highly heritable—environment has a great deal to say about how animals’ brains develop.
Then how useful is measuring heritability? For you the dog owner, maybe not very. For me the scientist, extremely, because if researchers can find highly heritable low-level biological traits, we can start working on finding the genes that affect them. Understanding the genes will lead to understanding the biology – and understanding the biology of traits like aggression or fearfulness will open so many new doors in how we prevent, manage, and treat these issues. Imagine understanding exactly what is different in the brain of a fearful dog, in terms of how her cells operate in different brain tissues, compared to a bombproof dog! This is the power of genetics, and well-designed heritability studies have the potential to serve as tools to help us get there.
So, to answer those questions:
“Are pit bull-type dogs genetically aggressive to humans?” Any dog showing aggression has some genetic risk of it. The right question to ask here is “Are pit bull-type dogs at higher risk of developing aggression to people?” Heredity studies would be a great way to approach answering this question, because such a study would also take into account the increased likelihood of pit bull-type dogs being subjected to abusive environments or simply poor socialization. To my knowledge, no such study has been done, meaning that labeling pit bull-type dogs “genetically aggressive” is not backed by scientific research. However, research has shown that poor socialization of dogs and keeping them as outside dogs is good predictor of aggression.
“Is my dog aggressive because of poor socialization (my fault) or bad genetics (the breeder’s fault)?” A genetic risk factor was present unless your dog suffered some truly awful environmental trauma. This doesn’t necessarily mean you should blame your dog’s breeder—behavioral traits are complicated to control through genetics. However, I do think that as a society we should be trying much harder to not allow dogs with unwanted behavioral traits to reproduce. We often prioritize looks or sport ability higher than personality! As for blaming yourself, remember that “environment” doesn’t just mean the dog’s current environment, but his past environment, all the way back to conception. That past environment may have been subject to bad luck (encountering something upsetting at just the wrong time in his development). So you’re not necessarily to blame, either. Sometimes bad behavioral traits develop and no one’s to blame. And other times dogs overcome difficult situations to grow into the soul of friendliness. That’s nature. And that’s nurture.
- Wilsson, Erik, and Per-Erik Sundgren. “The use of a behaviour test for selection of dogs for service and breeding. II. Heritability for tested parameters and effect of selection based on service dog characteristics.” Applied Animal Behaviour Science 54.2 (1997): 235-241.
- Ruefenacht, Silvia, et al. “A behaviour test on German Shepherd dogs: heritability of seven different traits.” Applied Animal Behaviour Science 79.2 (2002): 113-132.
- Strandberg, Erling, Jenny Jacobsson, and Peter Saetre. “Direct genetic, maternal and litter effects on behaviour in German shepherd dogs in Sweden.” Livestock Production Science 93.1 (2005): 33-42.
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