Tim asks why differences in ability are less unjust than differences in inherited wealth. Like him, I don't see much distinction. Having the right genes is as much a matter of luck as having rich parents. And even if ability is not genetic, it's still a matter of luck whether you grow up in an environment that fosters ability or suppresses it.
Indeed, I'd argue that it's luck that causes most differences in incomes across people. On top of the luck of having the right genes or early environment, there are (at least) four other types of luck:
1. The luck of being born at the right time. Teddy Sheringham has made vastly more money from football than Tom Finney. This has nothing to do with Teddy's ability, but everything to do with the luck of being born when footballers are well-paid. The same's true for vast numbers of skilled professionals and businessmen; they're lucky to be alive at a time when such skills are rewarded.
2. The luck of being born in the right place. Why am I in the richest 1% of the world's population? It's not because I'm in the cleverest or hardest-working 1%. It's because I'm English rather than Ethiopian. Herbert Simon has claimed that 90% of income in advanced economies is due to social capital, as distinct from individual abilities.
3. The luck of others' tastes and abilities. I make money as an economics journalist because others have a (queer) taste to read about economics, whilst few have the ability to do a comparable job. Almost all those who got rich by their "ability" are in the same position - they're lucky that others' tastes and skills are such that their skills are scarce but in demand.
4. The luck of chance meetings. Harry Markowitz says it was a "chance conversation" that led him to do the PhD dissertation than won him a Nobel prize. How many others owe their success to the happy accident of meeting an inspiring teacher, bumping into a man who becomes a great business partner, finding a job interviewer who likes the cut of your gib?
I reckon it's luck, then, that explains almost all differences in income.
The question is: should luck be pooled or not? Right libertarians have good (if not compelling) reasons to say not. But if they claim much more than this, they are just inviting ridicule.