[sdf4j]

> they all have the same mother, grew up in the same house, etc.

I’m pretty sure the first one didn’t have siblings, and the second only had one. Also their mother is not the same person after raising the first kid, or raising two.

Parenting never have reproducible conditions.

[adrianN]

I have twins (a boy and a girl) and you could tell they have a completely different temperament about two weeks after birth.

[leoedin]

I’m a twin - admittedly boy/girl, so already with some fundamental differences - and we are very, very different people. Always have been. Different interests, different ways of seeing the world, different attitudes to competition, sports, social relationships etc.

Now I’ve got 2 boys, and even at fairly young ages they were very different. I’d say by 6 months old the basics of their personalities were visible, and they haven’t changed vastly as they’ve grown.

[bugglebeetle]

The twins I have known are the same. I would assume it has something to do with a desire to differentiate themselves from one another, but they always seemed far more dissimilar in personality and affect than my siblings.

[riffraff]

OTOH could be siblings tend to be more similar as the smaller ones try to copy the older ones (my son would dress up as a ballerina to play with his sister when he was little, my smaller brother would acquiesce to play chess with me just to spend time together etc).

I have given up trying to explain child development, there's just too many variables.

[fhsm]

An adjacent point but despite ubiquitous birth order superstitions quality literature consensus seems to be that birth order is not a large driver of predictable differences. Example:

https://pmc.ncbi.nlm.nih.gov/articles/PMC4655556/

[SoftTalker]

Agree, those are some environmental differences. But any "microRNA" profile I might have contributed to the conception of each would be broadly similar. My life was pretty stable and levels of stress, diet, exercise, etc. were all about the same for all three.

[f1shy]

There are even identical twins that have different behaviors, characters and make different decisions.

[eipipuz]

I agree with what you are saying but remember the twin scenario. Spoiler alert, the kids are nonetheless different.