Hypothetical influence on alien reproduction on their social structure
This came to me when I was taking a lunchtime walk with some co-workers:
So, let's say there's an intelligent alien species with three sexes.
-A "fetus carrier" sex, that's analogous to human females: they get impregnated, and carry the fetus until birth.
-A "dominant inseminator" sex, that mates with the "carrier" sex. This sex is kind of analogous to human males. More on this in a moment.
-And a "submissive inseminator" sex. This is kind of like an "Independent" who carries the swing vote when there is a 50-50 split in the House or Senate among the two dominant parties. This will make sense in a moment.
When a "fetus carrier" and a "dominant inseminator" mate, the result is always a baby "fetus carrier".
In order to create a baby "dominant inseminator", a "fetus carrier" and "dominant inseminator" need to mate -- but during a fairly narrow time-period the "baby carrier" also needs to mate with a "submissive inseminator". So the "submissive inseminator" serves as kind of a catalyst to making a baby "dominant carrier".
When a "fetus carrier" and a "submissive inseminator" mate, the offspring is always another "submissive inseminator".
Note that this has somewhat of an analogy to humans: females carry the XX chromosome, and men carry the XY. I read somewhere that, really, "Males only exist so that they can make additional males." In other words, if females could figure out how to reproduce without intercourse, then two women could each contribute an "X" to produce a new female ("XX") with genetic diversity. So it would be a closed loop: females creating additional females.
With this in mind, maybe the alien "fetus carriers" are "(X)Y", the "dominant males" are "X(Y)Z", and the "submissive males" are "X(Z)" -- with the component in brackets being what is contributed during mating:
-"Fetus carrier" + "dominant male" = X + Y = another "fetus carrier"
-"Fetus carrier" + "dominant male" + "submissive male" = X + Y + Z = another "dominant male
-"Fetus carrier" + "submissive male" = X + Z = another "submissive male"
Another component is that "submissive inseminators" are physically smaller: about two-thirds the height of the other two sexes, and slender almost to the point of being frail. They also have about half the life-span of the other two sexes. To put it in human terms, the "submissive inseminator" typically has a lifespan of about forty years -- about the same time as the "fetus carrier" is near the end of her fertility. So it's almost like, biologically, they only exist to participate in the reproductive process.
So: How would this reproductive structure affect the social structure on that planet? Bearing in mind that on Earth, humans have basically one way of reproducing, yet have many different cultural social structures (e.g. pair-bonding; polygamy; polyandry (rare!); extended families; nuclear families; "free-love" situations; patriarchy; matriarchy) -- and some of these have changed over time, within a culture.
Because they are physically smaller, and have shorter lifespans, I predict that the "submissive inseminators" would generally be the less-powerful sex. However, the "dominant inseminator" needs the "submissive inseminator" in order to reproduce another one of "his kind". (Kind of like if, among humans, a male needed some sort of catalyst in order to have a son -- for those for whom that's an important thing.)
Because "submissive inseminators" and "dominant inseminators" do not mate with each other, they don't have to be attracted to each other. But both need to to be attracted to the "fetus carrier"
Likewise, the "fetus carrier" needs to be attracted to the two types of "inseminators" -- but probably in different ways.
Just as in most(?) industrialized cultures here on Earth, I would project that "dating" and relationships are primarily and at least initially for the "relationship" aspect -- and that reproduction only comes up later (although it can be an issue right from the beginning). So therefore I could see a "baby carrier" dating either a "dominant inseminator" or a "submissive inseminator" first -- and worrying about "having kids" later on in the relationship.
The cultural norm would probably be to have a three-person romantic relationship, and set up three-person households -- with one of each sex.
Among those who were out there, "dating", you would have two "layers" of being "single": being just an individual, looking for two partners -- or being a twosome, and looking for the third.
If you were already paired up as a twosome, it would be a little more difficult to find a third person that was compatible to you both.
You would have classified ads and dating sites with many different categories. Not including "same-sex" relationships (see below), among heterosexual folks you could have (to use human analogs):
-M seeking F or Z
-F seeking M or Z
-Z seeking M or F
-M and F seeking Z
-F and Z seeking M
-M and Z seeking F
Socially conservative people would probably say that a "marriage" (which would include three people, one of each sex) would be the best thing when reproducing and raising children.
Some people might have a two-person partnership, and only bring the third person in when they're trying to conceive -- a little similar to a same-sex human couple bringing in a third person when they want to have a baby.
There would be, mathematically, more possible forms of homosexuality: since being a threesome is normative, you could have a three-person same-sex relationship -- or two of one sex, plus one person of another sex.
Among humans, if one of the parents stays home part time or full time to look after the kids, it is usually the female. But occasionally -- especially if the female has a high-powered, good-paying job, the male might be the one to stay home. In this alien society, there is even **more** flexibility: although the "dominant inseminator" might be the one to typically work full-time, it might be about 50-50 for the "fetus carrier" or the "submissive inseminator" to stay home with the kids. So, similar to polygamous human relationships, there would be a greater opportunity for the "fetus carriers" to be "career women".
And probably other implications, as well...