6 ideas
Altruism between non-relatives can evolve when one organism helps another at a small cost to itself, with the expectation of being helped later at a comparable benefit. This requires repeated interactions, individual recognition, and the ability to detect and exclude cheaters who take help without returning it.
The sex that invests more in offspring (usually females through eggs, gestation, and care) becomes the limiting reproductive resource, while the lower-investing sex competes for access to it. This asymmetry predicts which sex is choosy, which is competitive, and the direction of sexual selection across species.
Because a parent is equally related to all its offspring but each offspring is more related to itself than to siblings, the offspring is selected to demand more parental investment than the parent is selected to give. This genetic asymmetry generates inevitable conflict over weaning, sibling resource competition, and the amount of care a child solicits.
Organisms that deceive others give off subtle cues that betray conscious lying, so natural selection favors hiding the truth from the conscious mind itself. By genuinely believing a falsehood, the deceiver eliminates the tells that would otherwise reveal the deception to observers.
Parents in good condition should bias investment toward sons because a healthy male can monopolize many mates and produce many offspring, while parents in poor condition should favor daughters who reproduce more reliably. The optimal offspring sex ratio thus shifts with the parent's resource status.
Behavior is best understood by asking what maximizes the propagation of the genes underlying it, not what benefits the individual, group, or species. This shifts analysis away from harmony and toward predictable conflicts of interest between any parties whose genetic interests fail to fully overlap.