The Climate Response Hypothesis

The Climate Response Hypothesis explains how human personality types evolved: in response to climate cycles in our species’ distant past.

The hypothesis details how our types evolved.

By reviewing recent studies in archaeology, anthropology, endocrinology, and climatology, the author of this site developed a hypothesis to explain why we humans vary so widely, personality-wise.

This Climate Response Hypothesis, as he calls it, describes how human personality types evolved during the Paleolithic. According to the hypothesis, that evolution occurred this way:

1. Cycles of climate change impelled humans to evolve different mating strategies, which gave rise to two basic human temperaments.
2. When climate changes enabled humans to live in larger groups, we evolved new social roles, which gave rise to eight natural human talents.
3. These two basic temperaments and eight natural talents combined to form 16 human personality types, which still exist today.

Climate cycles drove sexual dimorphism in humans.

During the past million years, the earth’s climate has fluctuated between wet and dry. In geological terms, it has done so frequently: every 20,000 years or so. What’s more, these changes occurred quickly, within the span of a few decades.

These cyclical changes in the earth’s climate complicated the lives of our hunter-gatherer ancestors:

This frequent and abrupt climate change rewarded quick thinking among humans, which was made easier by a bigger, more powerful brain. So, over many such cycles of climate change, human brains grew larger.

This large brain was an asset, but it came at a cost. The brain is an energy-hungry organ, which needed meat as well as plant foods to fuel (meat is much more energy-dense than plants).

Also, large-brained babies are difficult to birth and raise, so they were born increasingly “premature,” and needed years of care to become contributing members of a hunter-gatherer band.

These two characteristics of our species—an omnivorous diet and a long childhood—led us to develop divergent gender characteristics, which we know as masculine and feminine:

MASCULINE: HUNTING/FIGHTING	FEMININE: GATHERING/TENDING
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 kill, 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.

This dimorphism was effected through two main hormones.

How are masculine and feminine characteristics generated? Mainly through the hormones testosterone and oxytocin, respectively:

MASCULINE TESTOSTERONE	FEMININE OXYTOCIN
The main masculine hormone is testosterone, which increases muscle mass and bone density.	The key feminine hormone is oxytocin, which facilitates childbirth and breast feeding.
Testosterone also stimulates assertive behavior. It makes men more aggressive, and women more outgoing and sexually active.	Oxytocin also promotes trust and bonding. It makes women more caring, and men more cooperative and socially-oriented.

Everyone’s body produces both testosterone and oxytocin. That’s because early humans needed to embody both masculine and feminine qualities:

MASCULINE IN WOMEN	FEMININE IN MEN
Women used masculine structure to keep children safe, as when they set boundaries for them: “Stay away from that place. It’s dangerous.”	Men used feminine relationship sensibilities while hunting in a group, as when they cooperated to track and run down prey animals.

Humans evolved two different mating strategies.

Inbreeding makes humans less fit, which is why we’re pheromonally predisposed to avoid it. For nearly a million years, we lived in hunter-gatherer bands of around 25 people, so avoiding inbreeding meant mating with people from other bands.

Because our environment kept changing back and forth from forest to savannah, we developed a “mate with people from other bands” strategy for each:

FOREST: PROMISCUITY	SAVANNAH: EXCLUSIVITY
In forest environments, it was easy to avoid inbreeding: Bands occupied a smaller area because prey animals in forests didn’t migrate. Because there were other bands nearby, people didn’t have to travel far to find a mates in another band.	In savannah environments, it was difficult to avoid inbreeding: Bands occupied a larger area because prey animals on savannahs ranged widely. Because other bands were distant, finding a mate in another band required significant travel.
In the forest, increasing disease resistance was the main concern: Diseases flourish in wet climates, so to give children the genetic diversity that increases disease resistance, forest-dwellers mated with various people from other nearby bands.	On the savannah, avoiding inbreeding was the main concern: Suitable mates were harder to come by, so to ensure that mating with relatives didn’t occur, savannah-dwellers paired off with a single person from another distant band.

We see evidence of these two mating strategies in hunter-gatherer bands 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 East African savannah, and Venezuela’s monogamous Hiwi live in arid grasslands.

These strategies were effected through the same two hormones.

Our mating drives are generated by the parts of the brain that involve survival. This makes them too strong to be controlled by the “software” of a culture, so they had to be directed by the “hardware” of our genes. Thus the two human mating strategies—promiscuity and exclusivity—became genetically “hard-wired” within our ancestors.

Because these promiscuity/exclusivity genes had to persist through many generations, so they’d be available when the climate changed again, they worked through dominant and recessive pairs, like the ones that make us left- or right-handed.

Because the genes that directed human mating evolved in response to environmental changes, they had environmental triggers, like the genes for obesity that activate in a fetus when the mother is under extreme stress. For instance, in savannah environments, a pregnant woman’s body would have “told” a fetus: The climate is dry. If you have genes for promiscuity, turn them off.

Because the hormones testosterone and oxytocin produce the necessary effects—they make women more/less sexually active, and men more/less socially-oriented—these genes produced different set-point levels of those hormones in humans.

These hormones produce two basic human temperaments.

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

Thus, we humans have two basic temperaments: Sociable and Reserved. Expressed through our two genders, they work like this:

SOCIABLE	RESERVED
Sociable Man Born with genes that generate a higher level of oxytocin. This makes him more cooperative and social.	Reserved Man Born with genes that generate a lower level of oxytocin. This makes him less cooperative, less social.
Sociable Woman Born with genes that generate a higher level of testosterone. This makes her more outgoing and active.	Reserved Woman Born with genes that generate a lower level of testosterone. This makes her less outgoing, less active.

Climate change doubled the number of human social roles.

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

GATHERING Obtaining edible plants	FIGHTING Confronting other predators
TENDING Taking care of children	HUNTING Killing prey animals

Band members engaged in other social activities, of course, but these were the main roles that people were “hard-wired” to fill. For example, men with naturally high levels of testosterone were well-suited to Fighting.

Then, around 50,000 years ago, the earth’s climate became wetter and warmer, and stayed that way. Temperate environments that allowed more people to live in a given area expanded, so humans began living in larger groups:

As the size of hunter-gatherer bands increased to 50-150 people or more, their social dynamics became more complex, and maintaining their cohesion became more difficult. This problem was solved by the evolution of humans whose genetic/hormonal makeup equipped them to fill four more social roles:

DANCING Celebrating fertility/invoking spirits	PAINTING Representing life/providing meaning
SINGING Making music/storytelling	CRAFTING Creating useful/essential items

Today, we can see how these additional social roles help hunter-gatherer bands maintain their cohesion. Among the Agta, for example, people gifted at storytelling (Singing) are accorded high status. That’s because the bands that include skilled storytellers are more cohesive and prosperous.

These social roles were filled through the same two hormones.

Each of these eight social roles favored people with different inborn levels of testosterone and oxytocin. For example, a man with high testosterone was well-suited to Fighting and protecting a kill, while a man with high oxytocin was well-suited to Dancing and invoking the divine.

DANCING Women whose genes generated high levels of both testosterone (this made them more active) and oxytocin (this helped them go with the flow) were good at Dancing. This hormonal mix also made them particularly outgoing and relationship-oriented, and thus well-suited to facilitate the complex interpersonal dynamics within larger bands.	PAINTING Men and women with a lower level of testosterone (this made them more introverted) and a higher level of oxytocin (this made them more perceptive) were good at Painting. This hormonal mix made them inclined to do thoughtful work alone, and thus well-suited to illustrate the band’s shared values and meaning in cave paintings, on bone carvings, etc.
SINGING Men with moderate testosterone (this made them less aggressive) and high oxytocin (this made them more emotional) were good at Singing. This hormonal mix also made them congenial and creative, and thus well-suited to to coordinate the rituals and performances that burned off excess energy and kept people entertained.	CRAFTING Men with a lower level of testosterone (this made them more inclined toward creating things than killing things) were good at Crafting. This hormonal mix made them patient and productive, and thus well-suited to make the sophisticated stone blades that enabled humans to hunt and kill large prey animals.

Are we born with these predispositions? Yes. For example, if a person is exposed to more testosterone while in the womb, their ring finger will be longer than their index finger, and people with this trait predominate in sports that require aggression.

Our temperaments and talents combined to form 16 types.

Our species’ evolutionary past has equipped us with eight natural talents:

DANCING Moving in sensual rhythm	FIGHTING Using force to protect
SINGING Expressing feelings in sound	HUNTING Using force to provide
GATHERING Procuring good things	PAINTING Creating/appreciating art
TENDING Nurturing to grow and heal	CRAFTING Creating useful things

It has also given us two basic temperaments: Sociable and Reserved. For the sake of clarity, let’s use mythic names to designate men and women of each:

SATYR A sociable man	CENTAUR A reserved man
NYMPH A sociable woman	MUSE A reserved woman

Our two basic temperaments and eight natural talents combined to form 16 personality types, types that still exist today:

DANCING NYMPH Ardent and intuitive	FIGHTING SATYR Energetic and decisive
HUNTING NYMPH Lively and adventurous	HUNTING SATYR Steady and strong
SINGING NYMPH Sensual and expressive	DANCING SATYR Charming and creative
PAINTING NYMPH Sensitive and subtle	SINGING SATYR Smooth and balanced
GATHERING NYMPH Generous and friendly	PAINTING SATYR Imaginative and perceptive
TENDING NYMPH Nurturing and nice	CRAFTING SATYR Industrious and amiable
GATHERING MUSE Gentle and refined	FIGHTING CENTAUR Poised and protective
TENDING MUSE Kind and serene	HUNTING CENTAUR Focused and goal-oriented

Why do these particular types exist?

Why do these 16 types exist, out of the 32 possible options? For evolutionary reasons:

• Fighting Nymphs and Muses 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 natural testosterone to fill that social role.
• Hunting Nymphs exist because a higher-testosterone woman was useful to a band in two ways: in hunting small game to provide the band with much-needed meat, and in serving in domestic leadership roles (as seen with Clan Mothers in Haudenosaunee tribes).
• Gathering and Tending Satyrs and Centaurs don’t exist because men were too valuable as hunters, providing the band with meat, to have them spend much time gathering plant food and tending to children.
• Dancing, Singing, Painting, and Crafting Muses and Centaurs don’t exist because Muses and Centaurs evolved to procreate in small savannah-dwelling bands where the only social roles were Fighting, Hunting, Gathering, and Tending.