Personality traits Extraversion and Neuroticism were identified by Hans Eysenck in 1967, in 1975 he added Psychoticism. The currenctly most used personality model, Five Factor Model, or the Big Five (Extraversion, Agreeableness, Neuroticism, Conscientiousness, Openness), was proposed by Costa and McCrae in 1985. Eysenck though that personality traits have a neurological correspondence in the brain. For the current state of mapping of personality traits to neurological structures see Rachel L.C. Mitchell and Veera Kumari (2016)
https://www.sciencedirect.com/science/article/pii/S0191886916302628#bb0135
The mapping seems to be very complicated, which is not surprising as the brain is so interconnected organ. There is an easier way to show that personality traits are partially genetically determined: they are heritable to a rather high extent.
It follows that animals closest to humans should have rather similar personality traits. The two extant species that are closest to humans are chimpanzee and bonobo, both from genus Pan. Weiss et al (2000) gives the following personality traits for a chimpanzee:
- Dominance
- Extraversion
- Dependability
- Emotional Stability
- Agreeableness
- Openness
As Emotional Stability is the negative of Neuroticism and Dependability is almost the same as Conscientiousness, these are just the Big Five personality traits and one more, Dominance. Why dominance is not a human personality trait if it is one for a chimp?
For dogs there is the following list by Svartberg and Forkman (2002):
- Playfulness
- Curiousity/fearlessness
- Desire to chase
- Sociability
- Aggressiveness
Dogs display dominance, wolfs have a very clear dominance hierarchy, but humans always try to step into the position of an alpha male and the dog is pushed to a hierarchy level below all humans, this is why the trait does not appear in this list. Many traits in the list would not be considered as personality traits in humans. Playfulness would not be a personality trait for humans – puppies play but so do children. In both dogs and humans playfulness decreases by age. It is a general trait of young animals. Chasing seems like a hunting instinct. Humans have instinct like the sex drive and the mother instinct. As chimps occasionally hunt, human males probably have a weak hunting instinct. These instincts are not personality traits in humans, nor should they be in dogs if we look for corresponding traits. Aggressiveness is also not a personality trait in humans though aggression appears in all species including humans.
A possible mapping could be to associate aggressiveness with the negative of Agreeableness, Sociability with Extraversion and Curiosity with Openness. Neuroticism can be found both in humans and in dogs, though in dogs it usually means bad experiences in youth. Finally, Conscientiousness is a trait that varies in dogs. Service dogs are chosen from exceptionally dutiful individuals, who often belong to especially dutiful dog races. Some other dogs are lacking in this trait. Clearly, one can find this personality trait in dogs.
Thus, one can identify the Big Five personality traits also in dogs. Additionally there can be instincts, like chase-proneness in dogs, and dominance, which is not mentioned in the list for dogs but is there.
Some personality traits can be associated with parts of the brain. For instance, Neuroticism and extraversion are associated with amygdale. There is a connection between amylgade and fear: damage to amygdale has the effect that fearful situations no longer cause fear. However, amygdale seems not to be responsible for the feeling of fear, only detection of fearful situations and for generating the physiological response by releasing relevant hormones.
As aggression is often a response to fear, there is an association between aggression and amygdale, but it is not direct. Much more clear connection can be found between aggression and adrenaline. The gland secreting adrenaline is adrenal medulla and it is controlled by hypothalamus. Thus, male aggression has the closest association with hypothalamus while amygdale has a role in fear-fight-freeze behavior.
Amygdala is also involved in Extraversion. Other putative connection between personality traits and brain structures are explained in the Mitchell-Kumari paper, but my interest here is in dominance.
Dominance expresses as social hierarchy with alpha and beta members in animals who live in groups and as monogamous, polygamous, polyandrous, promiscuous or homosexual mating habits. The two closes species to humans, chimpanzee and bonobo, are very different in mating habits. Chimpanzee has a hierarchy, where every male is above every female. Mating is promiscuous and dominant males try to monopolize receptive females. In bonobos the highest ranking members of the group are females, though in two-to-two contacts bigger males usually dominate smaller females. Mating is promiscuous with many social homosexual contacts, especially between females.
It has long been a question what was the original human mating model. Gorillas and Pongos (orangutans) are polygamous. They are highly dimorphic: males are much bigger than females, and their 2D:4D finger ratios show high prenatal testosterone levels according to this article:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3081742/
Though 2D:4D studies sound like pseudo-research, they probably are not in this case. It is of course true that testosterone increases dominance and high testosterone is linked to polygamous life style. Concerning human degree of polygamy we know that humans have been fairly monogamous with some degree of polygamy, more polygamy in Africa, least in Asia and Europe in the middle:
https://www.sciencedaily.com/releases/2010/03/100302112018.htm
This study was done from X chromosomes, which means that the known and mysterious low human Y chromosome diversity does not influence the results. At least in fairly recent times humans have been rather monogamous. The earlier mentioned 2D:4D study reported that Australopithecus africanus also had small sex difference in finger length ratio and 2D:4D ratio, which suggests monogamy with some polygamous relations.
Humans developed from common ancestors with Pongo and Gorilla and at that time had high sexual dimorphism and polygamous mating model, but that was 8 million years ago. Around 5 million years ago humans diverged with genuses Pan (chimpanzee and bonobo) and Australopithecus. Genus Pan is not very dimorphic. If Australopithecus also was not very dimorphic (which can be disputed), early homo may have been only a little more sexually dimorphic than modern humans.
This conclusion is disputed, since Homo habilis seems to have been sexually dimorphic, but as
https://www.journals.uchicago.edu/doi/full/10.1086/667605
mentions, the bone fragments of Homo habilis may belong to two species and the identification of the larger as males and smaller as females may well be wrong.
Concealed ovulation in human females does suggest early monogamy
Intermediate size of human testes agrees with monogamy with some polygamy. Certain traits in human sperm, which are similar to Gorilla sperm, suggest that humans were mostly monogamous. This may be strange as Gorilla are polygamous, but the alternative is not between having a harem as a Gorilla male or a single wife, but between promiscuous mating as in Pan or mostly monogamous relations as in modern humans. Promiscuous mating would have caused sperm competition and sperm similar to that in Pan. Gorilla male faces no sperm competition as it protects its harem. We probably can conclude that early humans were almost as monogamous as modern humans.
This conclusion is in contradiction with the following paper from 2003:
which suggests that the reason for low diversity in human Y chromosome is caused by a recent shift from polygamy to monogamy. There was an effort to explain the lack of Y chromosome diversity by pruning of deleterious alleles:
http://journals.plos.org/plosgenetics/article?id=10.1371/journal.pgen.1004064
but it apparently this mechanism was not the reason. Male dominant lineages were also not the explanation, though also this mechanism can decrease Y chromosome diversity:
http://www.pnas.org/content/105/33/11645
The issue was finally settled in the 2015 paper by Monica Karmin et al:
https://genome.cshlp.org/content/early/2015/03/13/gr.186684.114.full.pdf
by suggesting a bottleneck in males around 8000 ybp, see the shocking graph in Figure 2.
Assuming that this study is right (and that the two mentioned earlier are wrong) starting of agriculture in around 10,000 ybp decreased the effective male population size for about 2000 years until around 8,000 ybp there were 17 mothers for every man who sired children.
The paper also refers to another article demonstrating that following the start of agriculture there was a population collapse in Europe:
These are very puzzling results and I cannot formulate a satisfactory scenario of what actually happened at the introduction of agriculture, but I will try.
Firstly, agriculture started at the same time in the Middle East with emmer, einkorn and barley and in New Guinea with taro. Later agriculture was apparently independently invented in many places. I do not find this scenario probable. It is more probable that the possibility of agriculture was known much earlier and maybe it was used for some plants, such as narcotic or medical plants, but there was no need for farming as long as meat was abundant. When many large animals went extinct, often because of humans, people had to turn to agriculture. It is not strange that agriculture would have in the beginning supported a larger population and then when fields were exhausted there followed a population collapse. This could happen.
The problem is the plots for women and men in Figure 2 of the 2015 article by Karmin et al. The effective population of women grew very fast before 10,000 ybp, while the effective population of men fell after 10,000 ybp. The number of reproductive women grows before agriculture started. Let us say that people shifted to polygamy around 16,000 ybp. The number of men stayed rather constant, while the number of women increased fast. That could happen if this population conquered another population, killed or enslaved the men and took their women, but this hypothetical population has not yet started with agriculture. Later they moved to agriculture to support the population and still kept the polygamous system, possibly women slaves. Thus, the master population decreased fast as shown by the plot with male effective population size. Finally the system collapsed or there was a reform.
As this development started before agriculture, there must be another change of economy. It happened at the same time when many large animal species went extinct. This must be causal: a group of humans invented an incredibly efficient way of hunting. This way was chasing animals over a cliff into a gorge, but it required dogs since humans are too slow to herd animals. First clear evidence of a tamed dog is from Germany around 14,000 BC, that is 16,000 ybp, which fits. Dogs separated genetically from gray wolves around 50,000-40,000 ybp, which fits to the time when modern humans moved to Europe. For some reason dogs were not bred for hunting and they looked like wolves for a very long time. This question is yet to be settled, but it seems like a new hunting technique using dogs was behind the population expansion. There had been an earlier population and the new population killed the men and took the women, which explains the ratio of 17 women to one 1 man in effective population sizes. Dogs played a major role in this conquest since weapons were not that different.
After large animals were killed to near extinction this population had to start agriculture. At the beginning they still managed to capture slaves, but later they did not. Children born to slaves were slaves. This explains why boys born to slaves did not have a chance to reproduce: they were kept as slaves and not allowed to mate. Without this explanation it is difficult to explain how one man to 17 women ratio can be kept for any longer time: men of the next generation rebel if they have the chance. Thus, they did not have the chance.
This is my scenario. Can this be a world-wide trend? Figure 2 in Karmin et al shows the same happening everywhere. Let us suggest: hunting with dogs spread everywhere very fast. Abundant meat allowed fast population growth. But to tell the truth, the scenario does not seem right. It must be something quite different. Figure 2 is a mystery.
What about dominance?
I think this is exactly a case of dominance and not connected with testosterone levels or brain structures. It is environmental as much of human variation always is. Anyway, the drop of effective male population size around 10,000-8,000 ybp and the fast rise of female effective population size from 16,000 to 10,000 ybp are mysteries.