• because of how costly pregnancy is, women have biologically evolved to be less sexually opportunistic (we have less sex, with less partners) - the opposite is true for men, because sexually opportunistic men would reproduce a lot, spreading and perpetuating those genes
• due to parental uncertainty, men would invest more in a child's well being if they think the child is theirs, even unconsciously - because of this biological evolution, fathers report higher emotional closeness to children that resemble them more (women don't, resemblance is pretty much irrelevant)
• female humans have periods because it allows our body to be selective about which pregnancies it invests in (because of how costly they are) - by basically flushing out weaker fertilised eggs

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• hormones are as important as neurotransmitters - both are chemical messengers, hormones belong to the endocrine system and travel through the bloodstream, whereas neurotransmitters belong to the nervous system and carry signals across the microscopic gaps (synapses) between nerve cells.