For most of the millions of people around the world working in countries far from where they were born, one imperative over-rides almost all others for their decisions to seek work elsewhere.
By Jeremy Scott-Joynt
BBC News Online business reporter
Unsurprisingly, it is about money.
The informal system may be under threat
But not necessarily, it has to be said, for themselves.
The armies of foreign workers - in Europe, the US and the Middle East in particular - are mostly taking the jobs locals do not want so as to put food on the table for their family back home.
This need to send money across borders to relatives and friends is driving a massive international industry.
Remittances from rich countries to poorer ones topped $100bn in 2003 - and they are only getting bigger.
With fees in 2002 of more than $12bn, according to World Bank figures, everyone is keen to get a piece of the action.
Western Union, for instance, estimates that the market can bear 300-500,000 officially-sanctioned money transfer agents worldwide.
Its spokeswoman, Danielle Jimenez, puts the firm's growth at 12% in 2003.
That's not simply grabbing market share to add to the 14% it already has. "The pie itself is getting bigger too," she says.
Under the counter
But the big competition for the official money transfer firms is not from one another.
It is from the informal route.
So-called "alternative remittance systems" - known in different parts of the world as hawala, hundi, chiti, chop and a thousand other names - exist everywhere that a diaspora can be found.
For centuries, they have offered a trouble-free way of moving value across borders, with minimal hassle and paperwork.
And they are not difficult to find either. In every major European or US city are import-export dealers, money-changers, mobile phone sellers and newsagents offering the service - at rates well below the official competition.
Harjit Sandhu, a 23-year veteran of India's elite Central Bureau of Investigation and then an adviser to Interpol, is well aware of just how easy it is.
Say you are an Indian working in New York, he explains. You see an advert in a local Gujurati newspaper for someone offering travel services and "great rupee deals".
That, it turns out, means getting money to your brother in Ahmedabad at a better exchange rate than the government will allow, and at a cost of no more than perhaps 3% of the overall sum.
You give him $1000. And the following day, his partner delivers the agreed sum in rupees.
But the money has never crossed a border. The partner in this case is a long-term business associate - although it could well have been a relative.
The trust relationship means there is almost no paperwork. Regulatory checks are non-existent.
The dollars stay in the US, the rupees stay in India, and any difference between what the two take in and pay out is settled via over-or under-invoicing in the import-export business the firm runs.
Another popular method is through gold deals in a neutral location - often Dubai, or elsewhere in the Middle East.
The result: A cost well below the 10-20% that many official channels will charge, a rapid service, and no legal quibbles - a valuable element given that many of the customers may well be undocumented migrant workers.
Of course, it is not only the bottom end of the employment ladder which is keen to stay unofficial.
For some countries there is no choice. Before Saddam Hussein's defeat in April 2003, for example, overseas Iraqis wanting to get money home would have to smuggle it across the Jordanian border via trusted intermediaries.
And the cost of official transfers makes this a popular option even where borders are more open.
"I'll do almost anything to avoid the banks," says one London-based South African professional - who, given strict currency regulations back home, prefers to remain anonymous.
She regularly sends funds home to help pay for a niece's schooling.
The cash route is one possibility. Paying money into friends' accounts in London, on the understanding of a matching payout at the other end, is another.
"I'm sending this money to my mum," she says. "I don't want her losing 15% of what I send - or her thinking I'm sending less than I said I would."
A new world
Now, though, the informal routes are coming under pressure.
Gold is one way of settling up
Although most of their use is for straightforward person-to-person remittances or business transactions, they can - and have - been used for laundering criminal money or for helping transfer funds for terrorism.
"At one end of the spectrum," Harjit Sandhu says, "it's a very useful community service.
"At the other, it's the perfect tool for criminals."
Before 11 September 2001, it was impossible for people such as Mr Sandhu to interest Western countries in hawala.
Now that has changed, and law enforcement and regulators are putting the squeeze on hawala dealings at home and on the dealers in destination countries.
The need to act is understandable. In a world where financial institutions are urged to know their customers so as to cut the risk of money laundering and terror finance, the undocumented, fly-by-night informal systems are a gaping hole in the defence mechanisms.
The pressure has already led to a doubling of official remittance flows in Pakistan, as money switches from informal routes to formal ones.
It also has costs.
One of the early actions taken post-9/11 to disrupt terrorist financing was the closure of al-Barakaat, a Somali hawala-style transfer service.
But Somalia, a failed state with no working central government, has no real banks. For the Somali diaspora, firms such as al-Barakaat were the only way to get money home.
For Harjit Sandhu, situations like this demonstrate the difference between what he dubs "white" - that is to say, remittance-based - and "black" or criminally-connected hawala.
The crackdown could well trigger collateral damage on the flow of remittances, not all of which the formal systems with their higher costs and regulatory burdens can absorb.
But others believe that since hawala and its fellows have been around for centuries, it will take more than the current regulation to stamp them out.
For one thing, getting a licence in, say, the US is hardly a challenge, one former financial intelligence officer says.
For another, the motivation to go informal is not going to go away.
A mistrust of officials and governments is as old as hawala itself.
And increased regulation simply adds to the imperative.
"Until someone gets around the problem of checking customers - the very thing that hawala people don't care about - there's going to be a big price gap," says Chris Hamblin, of financial regulation specialists Complinet.
"And the gap is going to get bigger as regulation increases."