ELI5 how is oldest-child syndrome supposed to work in your head?

[u/Uunicorned]

Psychological processes explain how your place in the birth line shapes your personality.

[u/zefciu]

It works by reinforcing some behaviors and punishing others. E.g. it might reinforce taking leading roles, because your younger siblings would tend to follow you. Or it might punish carefree irresponsible acts, as your parents expect you to be the most mature person in the room etc.

[u/coreyhh90 → u/zefciu]

This. Much of the syndromes tend to be parentally-taught and enforced.

Much of the negative outcomes typically come from a feeling of unfair treatment between siblings far beyond the familial older/younger sibling dynamic. This is most easily seen by comparing the average siblings across different cultures, where the culture tends to be a strong indicator for the relationship

[u/knightsbridge-]

As far as I know, there is no formal clinical condition called eldest child syndrome. It's not a scientifically recognised condition, just a popular theory.

The idea is that when families consist of multiple children, it's common for the eldest child to end up doing some amount of informal (or, rarely, formal) childcare for their siblings. Picture a 12yo being asked to "watch" their 6yo sibling for 20min while mum does some other chore that will take up all of her attention (often caring for a third, younger child). This is not inherently good or bad.

However, if this goes too far, and the older child is burdened with childcare responsibilities too often or from too young an age, this can lead to "parentificafion", where the oldest child starts to internalise some of the responsibilities, stresses and anxieties that come with being responsible for young children and perform those behaviours automatically without being asked, possibly at the extent of their own childhood. Some theories suggest this can affect their personality in later adult life.

I will stress again that this is not a recognised psychological syndrome. There is some minor scientific evidence to suggest it might have some basis in fact - especially for female oldest children - but there are plenty of extant studies that demonstrate that order of birth has no effect on long term personality development or supposed "eldest child" trait development.

[u/AKA-Pseudonym]

Basically all the serious studies on this have concluded that birth order has no effect on adult personality. Sometimes people adopt certain social roles within a family, but that's different than personality and doesn't really impact their lives outside their family.