The political genome is an extremely odd sequence. People who are born with it are rare in the extreme, and subject to some truly deviant forms of human behaviour.
Preparedness to subsist almost entirely on rubber chicken function meals, for one thing. Inability to turn mobile phones off. A prodigious capacity, in many cases, to remember one small fact about every single person they meet, so as to reserve the impression of great intimacy for deployment at the next meeting (''Margaret! How wonderful to see you again. I hope your whipper-snipper/cruciate ligament/shih-tzu is behaving itself?'')
In most, there is a vexed cocktail of idealism and mild to moderate megalomania.
But there is a shining, unmistakeable strand that is found only in the most successful politicians.
And that is the shamelessness gene.
In politics, you really need this one if you're going to get to the top. Without it, you're pretty much doomed before you even begin.
And as Kevin Rudd slipped back into the prime ministership on Thursday like a foot into a Birkenstock sandal, one was instantly reminded of his superior genetic profile for this line of work.
It takes a certain thickness of hide to call, after an extended period spent in gruesome hand-to-hand political combat with a personal and bitter enemy, for a ''kinder, gentler'' brand of politics.
And to decry the ''old politics of fear'', as a veteran not only of last century's anti-GST campaign, but also the 2007 alarum that John Howard's exploration of nuclear energy amounted to a planned reactor for every backyard … well. Life is long.
And the man Kevin Rudd displaced first time round was pretty shameless, too.
I don't think I will ever forget, for example, the 2001 press conference at which — after months spent explaining why it would be fiscally reckless to cut petrol excise — John Howard did exactly that. It was champagne shamelessness — weapons-grade shamelessness. There were plenty of episodes over his prime ministership; sometimes, you got the sneaking sense that he enjoyed them.
As for the man John Howard deposed … was Paul Keating shameless? Well, you'd have to say the 1993 campaign against John Hewson — in which Mr Keating deployed his considerable rhetorical firepower to defeat the goods and services tax he himself secretly agreed to be rather a good idea — is a pretty good indicator.
And Mr Keating's predecessor Bob Hawke, who is still probably wearing around the house the gold-plated commemorative underpants he won in the 1983 Summer Shamelessness Olympics, most likely needs no further discussion in this cramped patch of newsprint.
Tony Abbott's shamelessness manifests itself in a kind of naughty-chorister way. ''It's better to seek forgiveness than to ask permission!'' he grinned, for example, in 2009 when he went behind his colleagues' backs to announce the expensive paid parental leave scheme that most of them privately detest to this very day.
And sometimes, the truly shameless can even use their own disasters as a pole with which to vault merrily to higher ground. Former Queensland premier Peter Beattie — whose successful 2001 re-election campaign slogan was, in effect: ''I'm as disgusted as you are by the corruption in my government. Let's stamp it out together, shall we?'' — is an immortal title-holder in this particular art.
As for this country's immediate past prime minister … it's hard to tell.
Her decision to refer to her carbon pricing scheme as a ''tax'' — when it could conceivably have been argued that it wasn't — won her brownie points for plain speaking but exposed her to a lifetime's worth of broken promise taunts. A truly shameless politician would never have dropped the semantic point; witness John Howard's decade-long tussle with the common meaning of the word ''sorry''.
Kim Beazley didn't have the shamelessness pattern. Neither did Mark Latham, really — or John Hewson. True earnestness is inconsistent with the gene, and in Mr Latham's case, it's why he is a much better writer than leader.
There's nothing wrong with shamelessness, by the way, in politicians.
They have to have it, otherwise they either go mad or get bogged.
Sometimes, you need to be able to shimmy out of previously held opinions. Otherwise, you're trapped in them forever, even as circumstances change.
Without shamelessness, it's impossible to navigate through the archipelago of expectations that establishes itself more or less immediately around a new leader.
Without shamelessness, politicians would be bogged down permanently by obligations.
Politicians who can accept support from someone and then ''sell them for a cold pie'' — to borrow a Barnaby Joyce phrase — are not necessarily the most attractive human anthropological subjects.
But the alternative is an impenetrable culture of political patronage and mutual back-scratching — and that's not much fun either.
Annabel Crabb is the host of Kitchen Cabinet, which returns this Tuesday at 8pm, on ABC1.