• 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

2

• 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.

3

• high fertility (ovulation) makes us look, sound, smell, and feel sexier, men find us more attractive during ovulation
• the way our hormone cycle works, it makes us more prone to cheating, because during high fertility, we are more attracted to men with good genes (attractive men) who will ensure the health of offspring, especially if we're already in a stable relationship with a potential good father who will ensure the survival of offspring, because our instincts want the best of both world to have the best chances of passing on our genes
• during ovulation we are more attracted to “masculine men” (testosterone markers)
• this makes us unconsciously wary of ovulating women near our men, we get possessive, protective

4

• birth control tricks the brain into thinking our reproductive system is in the lutheal phase (where it basically stops working and waits for the egg to be fertilised), by telling it that both estrogen and progesterone are high so it doesn't need to do anything, therefore it doesn't order the start of another egg production
• the artificial progesterone in birth control pills (progestin) is usually made from testosterone (making it from progesterone is much harder), it's close enough to natural progesterone to trick the reproductive system, but it's not quite perfect, so women on the pill get some effects that testosterone gives
• dienogest is made from testosterone but blocks T receptors (anti-androgenic), avoiding T side effects
• but blocking testosterone has side effects of its own - like lower libido
• you can't send a targeted hormonal message to one part of the body and not to others, so there will always be “side effects” (aka effects, side effects are just the ones we didn't want, but they're all equally effects)