B-Y, I am posting this as a separate post because I am introducing a new and related term "millionaire's family".

It occurs to me now that a couple who has two children by design has not just two, but a boy and a girl*, which is the ideal combination, theoretically, for perpetuating the family line. The boy carries the family name, the girl the reproductive capacity. [* EDIT: I see now that that is exactly what you said in your original post, B-Y -- so it all fits. A "pigeon pair" is a "millionaire's family".]

Thus, a "pigeon pair" of children would be one boy and one girl, and that's it. That's all the parents intend to produce, perhaps for reasons of conscience in recognition of global overpopulation concerns [which China addressed, incidentally, by legislating only one child per family, and in rural areas, more until you produce a boy**].

This combination, one boy child and one girl child, is often referred to, for whatever reason, as a "millionaire's family".

In searching for some authority on this, I found the following Q & A on the Net:

Question:

When you have a boy and a girl, why do people say you now have a "millionaire's family"?

Answer:

Not familiar with this phrase, but I wonder if it doesn't mean that, now you have both an heir, the son to carry on the name and assets, and a daughter to be married off into another family, hopefully also with a moneyed and influential background, thus increasing your chances at power for generations to come. 'Bye!

http://snipurl.com/ddwt

For me personally, the answer provided above is not convincing. I think a "millionaire's family" is so-called because it is the ideal condition, the "gold standard", if you like, of producing offspring. [After all, if the motivation to produce a "millionaire's family" was to improve the parents' chances of marrying their offspring off into future generations of wealth and power, they would produce as many boys and girls as they possibly could, not just one boy and one girl.]

But, in recognition of the "Malthusian crunch", which, if not a "crunch", is still a threat to the sustainability of our planet, I think the "millionaire's family" should be called the "conscientous family" -- because having a lot of money really has nothing to do with it -- unless you plan to produce a dynasty, such as the "Kennedy dynasty". [Whether or not that is "conscientous" could be a matter of debate, depending on your politics. :) ]

** BTW an expert analysis of the implications of China's 'one child law', and how it affects China's future economic prospects compared to India's, can be found in a major Survey - "India and China: The Tiger in Front" - in the current issue of The Economist [on the newsstands now].

The portion of the Survey which includes this discussion is "premium content" but here is an extract from the Survey from Economist.com:

HOME to nearly two-fifths of humanity, two neighbouring countries, India and China, are two of the world's fastest-growing economies. The world is taking notice. In December, a report by America's National Intelligence Council likened their emergence in the early 21st century to the rise of Germany in the 19th and America in the 20th, with “impacts potentially as dramatic”.

http://snipurl.com/ddx5