When the law was changed to allow dual citizenship, the laws governing the birth of new citizens were also changed to prevent claims by tenth generation 'foreign' descendents.
I was born an American citizen ius sanguinis, and due to changes in legislation, I cannot automatically pass on citizenship to my child. I need to take the baby on a plane, go to America, and fill in the forms to get an expedited naturalisation. It won't take long, but there is one difference between naturalised citizens and citizens by birth - they cannot become President. A friend thinks I'm being slightly ridiculous worrying about this, but part of being American is the dream that anyone can become president. Including my child.
The 14th Amendment states that: All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the State wherein they reside.
Boris Johnson MP was born in New York City, so he was born American ius soli - and he can in theory be President.
Johnson wrote an article for The Spectator - That's it, Uncle Sam - dated 12 August, 2006.
He blogged it on August 29, 2006, about being turned back from "boarding a flight to Mexico, via Houston, Texas." We are always clearly told that US citizens have to travel on an American passport to enter the United States. The law has been applied for as long as I can remember. It's why I got an emergency passport issued by the Embassy last week. The main Immigration and Nationality Act, and all revisions, can be found here.
Johnson insists that "I am a loyal subject of Her Majesty, speak in an English accent, and for years I have travelled exclusively on a British passport. But my first passport was green" - old US passports were green. So in his blog he writes: "I make this formal, public, and, I hope, legally valid renunciation" of his citizenship, "I hereby renounce my birthright. Strike me off the list.". Umm ... no. You need to properly fill in lots of forms, and go to the Embassy - with a lawyer, to make sure you're not doing this on a drunken whim (see (a) (5) here; and this).
There are still many ways in which one can lose US citizenship if one wants to: for example Sec. 349. [8 U.S.C. 1481] (a) (2) taking an oath or making an affirmation or other formal declaration of allegiance to a foreign state or a political subdivision thereof, after having attained the age of eighteen years. I'm pretty sure this means that I can't stand for office in the UK.
Also important is: Sec. 349. [8 U.S.C. 1481] (a) (4)(A) accepting, serving in, or performing the duties of any office, post, or employment under the government of a foreign state or a political subdivision thereof, after attaining the age of eighteen years if he has or acquires the nationality of such foreign state. Until recently curatorial posts were civil service positions within the British Museum, so I couldn't even apply for a job there.
Pub.L. 99-653 clarifies that loss of citizenship can only be enforced by the US government if the potentialy expatriating acts are voluntary: trying to get elected an MP, applying to work for the British Museum ... For anyone who doesn't want to plough through the relevant laws, the US State Department has handy guides: Possible Loss of U.S. Citizenship and Dual Nationality; and Advice about possible loss of U.S. citizenship and seeking public office in a foreign country.
The IRS stuff we needn't worry about. The US and UK have an agreement to prevent dual taxing, and anyone who pleads poverty and cannot afford a house in their constituency, must surely fall within under the threshold.