Types evolved to help us survive.

Why do two hormones shape who we are?

Why do testosterone and oxytocin shape our personalities? According to the Climate Change Hypothesis, described in detail below, it’s because these hormones have played a central role in human evolution:

During the past million years, the earth’s climate has changed frequently, so humans evolved different testosterone-oxytocin profiles to help them survive those changes.
As hunter-gatherer bands got larger during the Late Stone Age, humans with different testosterone-oxytocin profiles evolved to fill important social roles within those bands.

Read on to learn more about this hypothesis, and its explanation of why human personality types evolved.

Climate cycles made human brains larger.

Fire-making humans of the genus homo have been around for about a million years. During all of that time, the earth’s climate has fluctuated between wet and dry. In evolutionary terms, these fluctuations occurred frequently: every 20,000 years or so. They also occurred quickly, within the span of a few decades.

These conditions challenged early humans. In East Africa, where they lived, wet eras created a forest environment. In other eras, the dry climate created a grassland environment. So, once they had adapted to living in grasslands, their environment would change to forest. Then, when they’d adapted to living in the forest, their environment would change back to grasslands.

These back-and-forth environmental changes rewarded quicker thinking, better problem solving, and more social cooperation, all of which were made easier by a bigger brain. So, over many such cycles of change, human brains grew larger and more complex.

Large brains led to large sex differences.

Our large brains were a survival asset, but they came at a cost. The human brain is an energy-hungry organ, and since meat is an energy-rich food (more energy-dense than plants), we needed meat to fuel it.

At the same time, large-brained human babies were difficult to birth, so they were born increasingly “premature,” requiring more time to develop outside the womb.

These two characteristics of our species—an omnivorous diet and a long childhood—led men and women to develop large differences in their sex-linked characteristics:

Men: Strong, Independent	Women: Flexible, Relational
Because men didn’t have to nurse babies or tend children, they were free to venture out and hunt game animals.	Because women nursed babies and tended children, they gathered plant foods with children close at hand.
Men provided band members with the high-calorie meat needed to fuel their physical and mental activities.	Women nurtured children during the long childhood needed for their physical and cognitive development.
To better hunt with weapons, and fight to protect the band, men evolved more upper-body strength and aggression.	To better bear, nurse, and raise big-brained babies, women evolved wider hips and more social flexibility.
Ranging afield and making plans to kill game, men became more independent and structured.	Interacting in domestic social groups, women became more sensitive and relationship-oriented.

These differences were effected through two hormones.

How are male and female sex-linked characteristics generated? Mainly through the hormones testosterone and oxytocin, respectively. These hormones enabled us to fulfill our Hunting/Fighting and Gathering/Tending social roles:

Hunting/Fighting: Testosterone	Gathering/Tending: Oxytocin
Testosterone increases muscle mass, and stimulates assertive behavior.	Oxytocin facilitates childbirth, and promotes trust and bonding.
Men have around seven times as much testosterone as women.	Women have around twice as much oxytocin as men.

Men and women evolved to have both hormones.

Men have oxytocin, and women have testosterone. Why is this the case? Because we humans needed to embody a range of qualities throughout our evolutionary history:

Men needed to bond with other men because it made them more effective in hunting as a group, cooperating to kill prey animals and defend their kills.
Women needed to be assertive to keep children safe, as when they set boundaries for them: “Stay away from that place. It’s dangerous.”

Humans developed two different mating strategies.

Inbreeding makes humans less fit to survive. We’re pheromonally predisposed to avoid it, but that can’t be done when mating within a typical Middle Stone Age hunter-gatherer band of around 25 people.

How did early humans solve this problem? By mating with people from other bands. Because their environment kept changing back and forth from forest to grassland, they developed a mating strategy tailored to each environment:

Forest strategy: Promiscuity	Grassland strategy: Exclusivity
In forest environments, it was easy to avoid inbreeding.	In grassland environments, it was difficult to avoid inbreeding.
In forests, prey animals don’t migrate, so bands in them occupied a smaller area. As a result, their members didn’t have to go far to mate with people in other bands.	In grasslands, prey animals migrated and ranged widely, so bands in them occupied a larger area. As a result, finding a mate in another band required significant travel.
Because it was easy to avoid inbreeding, increasing disease resistance was forest-dwellers’ main concern.	Because it was difficult to avoid inbreeding, preventing it was grassland-dwellers’ main concern.
In wet forests, diseases flourished. So, to give children the genetic diversity that increases disease resistance, forest-dwellers mated with various people from nearby bands.	In dry grasslands, suitable mates were harder to come by. So, to ensure that mating with relatives didn’t occur, grassland-dwellers paired off with a single person from a distant band.

These mating strategies were effected through our genes.

Our mating drives are generated by the pre-mammalian parts of the brain that involve basic survival. This makes them too strong to be fully controlled by the “software” of a culture. As a result, our mating drives had to be directed by the “hardware” of our genes.

That’s why the two human mating strategies—promiscuity and exclusivity—became genetically “hard-wired” within us. Here’s how that wiring works:

Our genes that direct mating induce promiscuity and exclusivity by generating different baseline levels of testosterone and oxytocin. Higher baseline levels of testosterone make women more sexually active, while higher baseline levels of oxytocin make men more sociable (an element of promiscuous mating).
These mating genes had to persist through many generations, so they were available when the climate changed every 20,000 years. How did they persist that long? Through dominant and recessive alleles, as in the genes that make us left- or right-handed.
These genes probably have environmental triggers, like the ones for obesity that activate in a fetus when the mother is under extreme stress. So, in dry grassland environments, the genes for promiscuity would be deactivated, to prevent inbreeding.

Mating genes produced four main categories of humans.

Because we modern humans inherited our genetic makeup from our hunter-gatherer forebears, each one of us is hard-wired to prefer either promiscuity or exclusivity.

Promiscuity involves sociability, and exclusivity requires reserve, so we’re born with one of two innate temperaments: sociable or reserved. Expressed through the two sexes, these two innate temperaments generate four main categories of humans. For the sake of clarity, the Teska Type System™ uses animal names to designate them:

Bear A man whose mating genes generate a higher baseline level of oxytocin. This makes him sociable.	Hawk A man whose mating genes generate a lower baseline level of oxytocin. This makes him reserved.
Fox A woman whose mating genes generate a higher baseline level of testosterone. This makes her sociable.	Dove A woman whose mating genes generate a lower baseline level of testosterone. This makes her reserved.

Early human groups had four main social roles.

In the small 25-30 person hunter-gatherer bands that we humans lived in for most of our history, there were four main social roles:

Fighting Confronting other predators	Gathering Obtaining edible plants
Hunting Killing prey animals	Tending Taking care of children

Band members engaged in other social and survival activities, of course, but these were the four main social roles that humans were “hard-wired” to fill.

Different social roles favored different hormonal profiles.

That hard-wiring evolved because different baseline levels of testosterone and oxytocin were adaptive for more than just mating:

Men with high baseline levels of testosterone were well-suited to Fighting. They could fight off large predators that threatened the band or tried to steal the carcasses of prey animals they’d killed.
Women with high baseline levels of oxytocin were well-suited to Tending. They could keep children safe, and help them become well-socialized so they grew up to be contributing members of the band.
Men with slightly lower baseline levels of testosterone were well-suited to Hunting. More patient and observant than higher-testosterone men, they could track down game animals that eluded others.
Women with slightly higher baseline levels of testosterone were well-suited to Gathering. More assertive than other women, they were equipped to go out, gather plant foods, and deal with what they encountered.

The number of human social roles doubled.

Around 50,000 years ago, the earth’s climate entered a wet phase. This created forest environments that enabled more humans to live in a given area, so we began living in larger groups. We also began migrating out of Africa en masse, so when the climate turned dry again, we still lived in large groups, in areas that retained their forests.

In forest and coastal environments, where food was abundant, the size of hunter-gatherer bands increased to 50-150 people. What enabled the size of these bands to increase? The evolution of humans hard-wired to fill four more social roles:

Dancing Expression through movement	Singing Expression through sound
Painting Representing life and meaning	Crafting Creating useful items

These new social roles were important to fill, because as bands get larger, they generate a new problem that must be solved: maintaining group cohesion.

Humans evolved four more hormonal profiles.

In hunter-gatherer bands, cohesion is paramount, because everyone must cooperate closely for the band to survive. As a band gets larger, maintaining that cohesion gets exponentially more difficult. That’s because there are four times as many social relationships in a 50-person band than there are in one that’s half as large.

Who can help a large band maintain its cohesion? Who can take specialized labor roles to make it more efficient? People with hormonal profiles like the ones below:

Bears and Foxes with oxytocin levels higher than others of their general types evolved for Dancing. Intuitive and relational, they were well-suited to facilitate interpersonal connections and reduce social tension within larger bands.
Bears and Foxes with oxytocin levels a little lower evolved for Singing. Congenial and attuned to emotional expression, they were well-suited to do the storytelling and mediation that fostered cultural cohesion within larger bands.
Bears and Foxes with more even levels of testosterone and oxytocin evolved for Painting. Less outgoing and more perceptive than others of their general types, they were well-suited to convey a band’s shared values and meaning.
Bears with oxytocin levels lower than others of their general type evolved for Crafting. Patient and productive, they were well-suited to make the sophisticated stone spear and arrow points that enabled hunters to kill big game more easily.

Band members engaged in a range of activities, of course. Men hormonally-wired for Crafting, for example, would engage in Hunting to help feed the band, and women wired for Gathering would engage in Dancing to blow off steam. Every member, however, had a natural talent in one general activity that made them particularly valuable to the band as a whole.

What is the evidence that this evolution occurred?

When it comes to the four main categories of humans, we see evidence of the two mating strategies that spawned them in hunter-gatherers today. The sexually-free Mbuti live in the Central African rainforest, and Colombia’s polyamorous Ache live in rainy river valleys. The sexually-restrained Hadza live on the dry East African savannah, and Venezuela’s monogamous Hiwi live in arid grasslands.

With regard to the eight general human social roles, we see evidence that humans evolved to take those roles in hunter-gatherer peoples today. Among the Agta, for example, band members who are good at storytelling (Singing) are accorded high status. That’s because the Agta bands that include skilled storytellers are more cohesive, and thus more prosperous.

The evidence extends to archeological finds. In settlements from 50,000 to 20,000 years ago, we see evidence of the increased social specialization enabled by new human hormonal profiles: finely-crafted spear points, delicate sculptures, and beautiful cave paintings.

Why are there only 16 personality types?

The four main categories and eight social roles combined to form 16 human personality types, which still exist today.

When we multiply four main categories by eight social roles, we get 32 options. So why, out of all the possible options, do only 16 personality types exist? For evolutionary reasons:

Women who evolved for Fighting don’t exist because fighting other predators was too risky for women—the procreative key to a band’s survival—to engage in. Women never evolved enough testosterone to fill that social role.
Men who evolved for Gathering or Tending don’t exist because men were too valuable as fighters, hunters, and craftsmen. It wasn’t efficient for a band to have them spend their time gathering plant foods and tending to children.
Hawks and Doves who evolved for Dancing, Singing, Painting, and Crafting don’t exist because their general types evolved in small grassland-dwelling bands. In these bands, the only social roles were Fighting, Hunting, Gathering, and Tending.
For similar reasons, Foxes who evolved for Hunting do exist. That’s because higher-testosterone women could be useful to a larger forest-dwelling band, in that they could safely hunt small game and take domestic leadership roles.