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Humans like many animals are extremely sexually dimorphic, well not really on the levels of Anglerfish but almost halfway there.

I wonder how would humanity be without Sexual dimorphism.

What if the only physical differences from motile(♂) individuals to Oogamy(♀) ones were sexual organs and nothing else?

And no things like breasts, feet, hair, lips, eyes, backs and hands are not sexual organs even if they can be attractive and have direct reactions to sexual stimuli

I'm looking for answers related specifically to effects on societies and how different would human history and present be.

And hopefully this doesn't offend anyone but, I'm curious. Would things like transsexualism exist? Or would sexual attraction not exist, would people be attracted to other people or simply be attracted by the idea of having sex?

Ulterior details:

When needed, each of the sexes can gain the ability to breastfeed, not interested if this involves to have permanent wide fatty tissues over the boobs or growing it only when necessary like most animals do, I don't know enough about evolution to tell which way would be most preferred by effectiveness/energy cost.

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    $\begingroup$ Do you believe in the hypothesis that there are psychological differences between men and women which are not purely based on socialization? If yes, does the dimorphism also apply to psychology? $\endgroup$ – Philipp Aug 11 '16 at 19:48
  • $\begingroup$ I asked precisely for only physical differences in sex $\endgroup$ – Charon Aug 11 '16 at 19:49
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    $\begingroup$ You are aware that most of the gender distinctions in humans, including many of the items you list (feet, hair, lips, eyes, backs, hands) are cultural, not innate? $\endgroup$ – cobaltduck Aug 11 '16 at 19:54
  • $\begingroup$ Some clarification is needed here. For example, breasts have an obvious and essential role in the postnatal stage of reproduction. Do these monomorphic humans not breast-feed, or do breasts only form when lactation is needed? $\endgroup$ – rek Aug 11 '16 at 19:57
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    $\begingroup$ @rek That's not correct. Mammary glands have a postnatal function in mammals. But the lumps of fat which accentuate them in such a visually appealing way are unique to humans. $\endgroup$ – Philipp Aug 11 '16 at 20:05
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First, we need to consider why sexual dimorphism exists in the first place.

The first reason is because different sexes have bodies which are optimized for the different roles they have when it comes to caring for children. For example, in bees the sexually active females (queens) have much larger abdomens than the others. That's because they lay hundreds of eggs each day and don't do much else, so their bodies consist mostly of their reproductive organs. Spiders and some insects kill and eat the male after coitus to get the nutrition they need to make eggs, which requires that they are much stronger than the males.

Lack of gender roles wouldn't be effect but cause for evolving no sexual dimorphism, so we can assume that gender roles would be next to nonexistent.

A second reason for dimorphism is that it makes it much easier to find a partner. Most species (including humans) have little dimorphism as long as they are preadolescent and only develop sexual characteristics when they become sexually mature. So looking for people with the right sexual characteristics helps to find a partner who is a) of the right sex and b) sexually mature enough to procreate with them.

That means a species without dimorphism would need another way to separate partners from competitors.

An easy cop-out would be pheromones. But then we again have a different perception of males and females which appears to be against the spirit of this question.

One way would be a trial-and-error courtship behavior where everyone tries to get into the pants of a lot of different people until they find the correct equipment in them. This would be even more tedious and frustrating than our courtship behavior, so we can assume that emotional bonds won't form until finding out the sex of the partner.

Another would be to actually reintroduce gender roles. Sexually mature and available people will behave differently depending on their sex, which would allow to recognize them as such.

Would transsexuality exist? Good question. There is no consensus about why transsexuality exists in the first place. Also, there is no such thing as the "typical" transsexual. There are many different reasons why people feel that their biological sex doesn't fit them. Some trans people don't even know the reason themselves. But fact is, the less difference there is between sexes, both physically or culturally, the less there could be one could feel is wrong when they have the wrong biological sex. So when there still is transsexuality, we can assume that it would affect the daily lives of transsexual people a lot less.

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  • $\begingroup$ Note the blue-ring octopus. I saw on some nature show that they are so rare and solitary that it will mate immediatly with any other individual it encounters on the chance that it’s the opposite sex. $\endgroup$ – JDługosz Aug 11 '16 at 20:44
  • $\begingroup$ No sexual dimorphism doesn't mean no gender roles. I think you are making a leap there with little justification that is really quite important to the question of how societies would function. Females will still bear children and males won't. That's a fundamental difference. With regards to telling the sexes apart, might individuals just not wear pants? Males still have exterior genitalia right? Trial-and-error courtship wouldn't even be all that tedious, look at sexual activity in Bonobos for instance where sex between various pairings happens quite frequently. $\endgroup$ – Mike Nichols Aug 12 '16 at 15:37
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Not much would change.

Without the instinct to reproduce, humanity would not exist. Both the sperm-producing (let's call them "men") and womb-equipped ("women") humans would be looking for a successful mating, so they would:

  • Act and dress differently in order not to waste time wooing individuals with the same sexual organs.
  • Look for different, more subtle signs of health and ability to provide for offspring than broad shoulders or wide hips.
  • Maybe have a more equal division of labor, as the men don't have more strength and aggression than women, so there is less reason for women not to participate in hunting and war when needed.

It's difficult to say anything about sexual orientation, because we don't know the exact mechanism by which the brain develops one.

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