I have 5 kids and I can also attest to this. Each one is different. In our case, the children are in two personality groups: the 1st and 3rd child have a similar personality type, and the 2nd and 4th have a similar personality type that vastly differs from the first type (the 5th is too young to really be expressive right now). Each child is unique in their own way, and they all respond to different risk-reward profiles.

Before I had kids, my grandmother, who had 11, tried to tell me that you just couldn't do much about your kids's personalities. I was naive and didn't believe her until I had 2 and saw the difference first hand.

I am NOT trying to downplay the importance of nurture, but parents have a larger challenge than many non-parents or single-parents expect. It's not just about formulating a good environment (in fact, formulating an environment that is too easy is probably a substantial developmental detriment). Your kids come out the way they are, and you have to really understand their personalities and challenges and then teach them ways to cope with the world while accepting that you can't change their core internal traits. It's an intellectually and emotionally intense labor of love that lasts for decades, and it shapes both parent and child.

I have four daughters. Can confirm.

Well, there's also the fact that unless you are homeschooling and preventing your children from experiencing human and media contact, your nurture input is only part of a whole. I learned a lot of what I did and did not want to be like from my parents growing up, but I also learned a lot from my siblings[1] and my friends, and my school experiences.

That is, you may provide the same ingredients, but ultimately the kid's the baker, and you're not even really in the kitchen to see what goes on. Who knows what makes it into the final product?

1: Aside from the normal older brother semi-worship for a while, I also witnessed what sounded like him coughing up a lung every morning for a winter or two with chronic bronchitis from smoking. Peer pressure to smoke (and there was quite a bit, most my friends did) isn't quite the hurdle you might think it to be when you have that in mind.

Except the differences start showing up long before school age.

And upfront, they were all treated differently. Not by fault, mind you, but just by happenstance. You probably didn't fawn so much over the second, and even less on the third. Not because they are less special, but more because you had a sense of what you are doing. You came into the second and third child with a battery of things to try because you tried some on your first child.

You are probably less strict with the youngest child and worry a bit more over the oldest.

Plus you and your spouse (or whoever) have aged and matured with time, so each child gets to know a slightly different version of you, your spouse, your parents and family. The more spaced out your children are, the more this will become so.

Well sure. Notice I said homeschooling and preventing most contact (which actually makes homeschooling redundant).

Kids likely won't turn out the same even with the exact same environmental inputs, but I'm just pointing out that we often vastly over estimate our ability to control those environmental inputs in the first place, so the point is somewhat moot.

Although to be fair, having older or younger siblings probably changes your boys a lot too.