The lesson of the last century, as I see it, is that society is shaped by people, and people are shaped by evolution, and evolution is frequently an irritating bastard that won't let you do what you want, no matter how bad you want it. It's all well and good to say "there should't be any prejudice or hate and we should love everybody," for example, but in fact the reason we've got prejudices of any sort is that over the last 3 million years it's been a survival trait: Those with prejudice survived ("Hm. Those people in the next tribe killed and ate my friend. Probably I shouldn't ask 'em for help...") and those without didn't. ("Hm. Those people in the next tribe killed and ate my friend. Probably, I should go over and try to understand them and see the situation from their point of view and extend the hand of universal brotherho----yipe! Ow! Ow! Ow! At least brush your teeth first!"). Prejudice is neither good nor bad, it's a tool. What you do with it is good or bad. A little prejudice will save your life and protect your kids, a lot of prejudice will make everyone's life much, much worse. Racism is bad. A tiny bit of skepticism about illegal aliens from Canada is good.
We're told that tolerance is always good, and intolerance is always bad. Nonsense: it depends on what you're tolerating. I'm utterly intolerant of racism and child abuse. That's a good kind of intolerance, right? Tolerating pedophilia is a bad kind of tolerance, right? Not tolerating Jews living in your neighborhood is a bad thing. Tolerating drug pushers in your neighborhood is also a bad thing. Again: it's a tool.
Or the whole shrill atheism thing where people get really, really, really mad when Christians (Or anyone else handy, but mostly Christians) express faith in non-rational non-scientific stuff. "Religion is the cuase of all the suffering in the world." Bullcrap, sparky: People and earthquakes and bad weather are the cause of all the suffering in the world. But mostly people: It wasn't the Dali Lama that killed 30 million people in the Soviet Union. Religion is just one of the many things people do to try and make sense of life, and bond with their neghbors. Why? Because 3 million years of evolution had bred it into us: religious people form societies easier, will put up with more personal crap for the sake of the whole, try to help people out, and tend to hold on when there is literally no hope. They also tend to be more stable in family situations. This is why every society has some kind of religion: Because belief in hope is a survival trait. Our fundamental wiring is geared for it.
The 20th century was a time of arbitrarily saying "none of this stuff has any bearing on us, so let's just get rid of things we don't like and remake the world in any willy-nilly way that seems trendy." Well, fine, you don't learn stuff if you don't experiment, that's swell, no harm no foul, but as a general rule of thumb you ignore three thousand centuries of evolution at your own peril. It's a kind of stupid thing to do, you know?
So you can change the way a society USES prejudice, but you can't get rid of it. You can transform the role of religion in society, but you can't get rid of it. Attempts to do so will just cause frustration and psychological stress that further destabilize society, and ultimately the thing you're trying to get rid of will crop back up anyway, because it's in our blood.
Thing is: since we're human, we have certain needs and roles we still have to fill in order to function, or else society doesn't work.
Start with the family: Dad, mom, kid. It is not written in our genetic code that dad has to be the breadwinner, and mom has to be the nurturer, but it *is* written in our code that *SOMEONE* has to fill those roles. Let's say you're a gay family: maybe that'll work out for you, maybe it won't, but for the benefit of the kids *somebody* still has to fill the 'breadwinner' and 'nurturer' roles. Someone's got to be the supporting, understanding one, and someone's got to be the cop. Let's say you're straight: you *still* need to fill those roles. Thirty years ago, a lot of my friend's parents were into the whole "I don't want to be an authoritarian like my parents were, I just want to be a pal to my kids." Then they'd sit around smoking pot with 'em. No, really, it was kind of trendy for a while. So the kids mostly grew up to be ineffectual, inarticulate emotional wrecks with a muddled moral compass at best, and utter wastes of human skin at worst. Many fell into cults and gangs, craving the guidance their parents never gave 'em. Those that prospered generally did so in spite of their upbringing, not because of it. Those that didn't prosper have - for the most part - gone on to do an even more disastrous job with their own kids, split homes, baby-daddies, it's a mess. It's a consummate shame. It's a national disgrace. No, it's worse: it's a disgrace to the species itself.
Why? Because humans are programmed to need people in certain roles while they're developing. If they don't get, they get all messed up. Pleasant or unpleasant, nature doesn't care: we are a somewhat hierarchical species of pack-hunting predatory omnivores. You can pretend that throwing money and platitudes at social ills will change this, but it's really naive, and kind of unscientific. And ultimately, kind of destructive, as my somewhat tendentious illustration shows.
The lesson of the 20th century, and the largest failure of liberalism is the idea that we are entirely what we choose to be. The overlooked lesson of conservatism is that we can indeed be whatever we choose to be, but our choices are always gonna' be dictated by our past.
Because really what else is there to base a choice upon?