What is it about the daughters of America's elected monarchs and monarchs-in-waiting, when they arrive in Washington DC? Go through the 16 men who have been either president or vice-president since 1961 – from the era of John F Kennedy to that of George W Bush – and their sons actually outnumber their daughters, by 27 to 26.
But once a family gets close to the white stucco mansion at 1600 Pennsylvania Avenue, or No 1 Observatory Circle, the vice-presidential residence just across from the British embassy here, it's the daughters that make the news. And that pattern will not change, whether Barack Obama or John McCain becomes the 44th President next January.
The question is: does having a daughter have an impact on a candidate's rating? Is there such a thing as a "blonde bounce"? Can a rebellious teenage girl reveal weakness in her father? The number of column inches generated by daughters would suggest that they are key players in their fathers' campaigns.
It began with Caroline Kennedy – now a 50-year old Democratic grandee who helped Obama pick his vice-president this year – when she was photographed at five or six, romping around with her father and infant brother, John Jr, in the Oval Office. Since then, daughters have made the running. Tricia Nixon got married at the White House in June 1971 (unfortunately just as The New York Times leaked the Pentagon Papers, which spoiled the day for the father of the bride).
Three years later, Nixon resigned over Watergate and rebel teen Susan Ford moved in with father Gerald. She wore jeans, brought home dates, and eluded her secret service protection to escape, unchaperoned, on to the Washington streets. "I got out," she later remembered, "and thought, 'Oh crap, what have I done?' I picked up one of my friends and we drove around and went to a couple of bars." Then she remembered she was going to a rock concert that evening, and the secret service had the tickets. Sheepishly she went home and apologised to Dad. All was forgiven, and she didn't miss the concert.
Amy Carter, Jimmy's daughter, was nine when they arrived in 1977 and, like Susan, she found the place stuffy and confining. It didn't help when the media discovered her reading at a state dinner – or that she had 39 teddy bears.
It was 12 years before another young girl called the White House home. Chelsea Clinton went to a local high school and tried to lead a normal life. The press broadly observed a gentleman's agreement with the Clintons not to write about her.
Next came the Bush twins, Jenna and Barbara, 20, students at Texas University when George W became President. They played hide and seek with the secret service, but, by his second term, Laura and George were empty-nesters.
Now new First Daughter trouble may be on the way.
An Obama victory would see his children – Malia, 10, and Sasha, seven – as the first pair of youngsters in the White House since JFK. If McCain wins, Bridget, 17, his adopted daughter from Bangladesh, will live there.
Even potential VP Sarah Palin's family drama, however, will probably have little impact on the great sweep of history. Family matters rarely do. No political pundit has yet suggested that wider knowledge of Amy's 39 teddy bears might have staved off Carter's rout at the hands of Ronald Reagan.
Obama brings Malia and Sasha to many political meetings – but in the polls he's running little better than even with John McCain, who until this week kept all his children out of the limelight.
And then there's Dick Cheney. As father to Mary, who lives with a lesbian partner, Heather Poe, and son Samuel David Cheney, he has been an exemplary, understanding modern dad. But that hasn't stopped him being the most unpopular vice-president in modern times. Public vices outweigh private virtues.
USP: Pregnant, unmarried.
She said: Nothing. It's more what she did.
Mom said: "We're proud of Bristol's decision to have her baby [boyfriend Levi Johnston is the father] and even prouder to become grandparents."
Vote winner? Amazingly, what on paper looks like a Hollywood soap opera is playing well. (Interesting siblings: Track, 19; Willow, 14; Piper, seven; and baby Trig, four months.)
USP: Rescued from Bangladesh (John didn't know) by second wife, Cindy.
She said: Nothing. Wiped a tear at the convention.
Dad said: "Where is she going?" when he saw Bridget; recovered to say, "She has enriched our lives. She's a wonderful child, a complete part of our family, and we love her."
Vote winner? Meghan's syrupy book for kids, 'My Dad, John McCain' could win over a few "soccer moms", but Bridget is the ace for tugging at America's heartstrings.
Sarah, Kristin and Karenna Gore
Ages: 29, 31, 35
She said: Karenna: "Welcome to the next president of America and a fine dad."
Dad said: "When our first daughter, Karenna, was born, I began to see the future through a fresh set of eyes. I know a lot of you have had that feeling, too. And I decided that I could not turn away from service at home – any more than I could have turned away from service in Vietnam. That's why I ran for Congress."
Vote winner? To the great sadness of many, Gore's girls couldn't beat the hanging chads.
Ashley Blazer Biden
USP: A rebel, she obstructed police in an altercation outside a bar six years ago. Now she's a social worker.
She said: "Don't you know who my father is? He will have your badge for this," on her arrest.
Dad said: Pointedly chose son Beau to introduce him at the convention, and said: "Beau, I love you. I am so proud of you... And I'm so proud of my son Hunter, my daughter Ashley."
Vote winner? Er, no. But the memory of Biden's first daughter, Naomi Christina, who died as an infant with his first wife in a car crash, is a humanising force in an election in which children are key.
She said: "I love my father. I don't want to be a distraction."
Dad said: "You're my daughter and I love you and I just want you to be happy."
Vote winner? She was seen, in 2000, as offering a "compassionate conservatives" angle to the Republican party, and has subsequently appeared in support of her father at official events. Raising a son with her long-term partner has infuriated such groups as Concerned Women for America. (Less interesting sibling: Elizabeth, an attorney, 42.)
USP: Child as marriage "Band Aid"; candidate-in-waiting.
She said: "What I would like America to know about my parents is they're great people." Or: "I'm sorry, I don't talk to the press, and that applies to you, unfortunately – even though I think you're cute" (to a nine-year-old school magazine reporter).
Dad said: "If you asked me [if Chelsea would run for office] before Iowa, I would have said, 'No way. She is too allergic to anything we do.' But she is really good at it."
Vote winner? As the victim of a parentage joke too tasteless to print, plus her sheer resilience in the face of attacks on the family, she's now an asset talked of as a future politician herself. Watch this space.
Natasha 'Sasha' and Malia Ann Obama
Ages: Seven and 10
She said: Malia: "They just think I'm cute, so I wave and smile, and then I'm out of there." Sasha: "I think she did good," on mother Michelle's convention speech.
Dad said: "[I am] aware that my parents' dreams live on in my precious daughters."
Vote winner? How could they not melt the hearts of American citizens?
Jenna Welch and Barbara Pierce Bush
USP: They're twins
She said: Jenna: "We kept trying to explain to my dad that when we are young and irresponsible, well, we're young and irresponsible."
Barbara: "I know it's hard to believe, but our parents' favourite term of endearment for each other is 'Bushy'."
Dad said: "I told the girls when they were young – don't worry, we'll go on the famous father-daughter camping trip... And this is the grand camping trip we never took them on, campaigning for president."
Vote winner? If Bush couldn't control two young daughters, what hope was there?
For rolling comment on the US election visit: independent.co.uk/campaign08