- Sex - this instinct, or impulse, or whatever you call it - has is a reasonable impulse, for by it the species is continued.
And as it is impulse based, getting pregnant is not always well planned. Furthermore even if it is well planned, circumstances can change in 9 months. So at the time of birth it could be realized that this pregnancy was an error.
While this is not important on an individual scale, this is. You who are currently young and virile, will get older. And if you have offspring, you will have someone young and virile to support you in your elder years.
For the offspring to support one, it must be the right number, with the right timing, under the right social cicumstances and of decent quality. If for example its female and male would be preferable for supporting, discarding the female and get pregnant again soon is more efficient, than keeping the female alive, as it might prolong the time to the next pregnancy and it will cost resources not avaible for the hopefully next male child.
Or the social circumstances are such, that the long term prospects are reduced, e.g. pregnant although promised to some “good” husband and the quality of avaible husbands is reduced if someone knows about the child (therefore auctioning the child is not an option).
So on the individual level a lot of survival optimizing reasons for infanticide exist.
For the whole society infanticide is only a problem, if it happens to often. The romans build an empire, while infanticide was legal.
- There is also a natural impulse for survival. We do not do things that make survival less likely.
No, survival instincts only work, in case of direct danger. In case of indirect or long term danger (e.g. smoking) survival instincts have reduced importance.
- Considering 2 and 3, it is logical to conclude that infanticide goes against our basest instincts, and therefore is, from a natural standpoint, counter-productive to survival and thus wrong.
If survival requires certain qualities, which can be identified at birth, infanticide is beneficial for survival, as the resources wasted on unfit off spring is reduced, this was the approach of Sparta.
The resource investment between birth and productive age is big compared to the prior investment. So opting out of the deal suggested by the child (feed me and you can educate me into being your supporter later) can be rational.
Afterthoughts:
therefore, by this postulation, it makes more sense to have more and well-raised children who can aid you when you are aged and less able to care for yourself. Abortion, contraception, fornication, and sodomy are all counterproductive to natural human survival, and indeed are the bane of some our most basic survival instincts.
Lets discuss infanticide only. While more and well raised children are good, for this prohibition of infanticide is not necessary, the romans objectively had enough sons to conquer the mediterran world, as the greeks had enough sonds 200 years prior to conquer persia.
Furthermore the need for “quality” children would indicate infanticide of handicapped children is preferable.