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.

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bugglebeetle 6 hours ago | parent [-]

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 6 hours ago \| parent [-]
		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.