London - Booking a holiday is unlike other big purchases.
The customer makes a significant financial commitment, usually months ahead, and enters into a contract that it is typically expensive to break - from the holidaymaker's point of view, at least.
As departure date approaches, events may trigger “buyer's remorse”, with the prospective traveller wishing they had booked to somewhere else, or indeed not booked at all.
Right now millions of families across the world will be facing that reality, as they contemplate events since July 14 in the light of their impending holidays. They are quite likely to discover that their travel companies are enforcing their terms and conditions.
Tour operators are generally entitled to refuse refunds and transfers unless there is government advice against travelling to a destination - as is the case for Tunisia. And even if the Foreign Office advises against travel to a destination, airlines are within their rights to say, basically, “Our flight is still operating, and the fact that you no longer want to go isn't our problem - it's yours.”
But what, in the short term, is the holidaymaker's problem will soon become a concern for the whole travel industry - with major implications for an important slab of the economy both in Britain and abroad.
The UK vote to leave the EU, which took place three weeks before the atrocity in Nice, had already weakened confidence. Travel firms have been reporting slow sales of “lates”.
These are the airline seats and hotel beds that are subject to who-blinks-first bargaining, with holiday companies keen to sell their distressed inventory at the highest possible price, while prospective buyers in need of sunshine take a view on whether the price will fall or the trip will be snapped up by someone else.
At this time of year - a week away from the end of term for many schools in England and Wales - holiday firms are usually confident that last-minute packages will sell profitably.
But this year, such certainty is dwindling. If bargain-hunting Brits were already wavering because of the slump in sterling since the Brexit vote, events in France and Turkey may persuade them to stay at home.
When that means holidaying in Britain, at least it is good news for the UK tourist industry. But for some it will literally mean remaining at home - keeping their cash in their pockets, while surrendering all the personal benefits that derive from travel.
With so much strife elsewhere, Britain seems relatively safe. When you look at risk dispassionately, the UK is indeed one of the safest places on the planet, largely because of our very low road-accident rate. Yet when people are deciding where - and whether - to travel, their perception of risk is often deeply flawed.
They start with the journey itself. Typically a traveller will rate the flight as the most dangerous part of the journey to their destination, even though it is safer - by several orders of magnitude - than a road journey to the airport.
Point out to an anxious passenger about to step aboard a British Airways or easyJet flight that no UK passenger jet has crashed with loss of life since the 1980s, and they are unlikely to feel reassured. And when that traveller reaches their destination, they may continue to apply willful ignorance of probability.
He or she will cheerfully hire a car and drive it on unfamiliar roads in the company of motorists who are statistically far more likely to crash than those in Britain. The roads in Spain, Cyprus and Malta are around one-third riskier than the UK. Turkey, France, Italy and Canada suffer death rates more than 50 per cent worse. Things get progressively more dangerous in Portugal, Greece, Croatia and the US.
Yet human nature tells them they are “in control” and that, therefore, their risk exposure is minimal. But even in a year such as 2015, in which 30 British holidaymakers were murdered on the beach at Sousse in Tunisia, 10 times as many UK travellers perished on foreign roads. A holiday is an emotional purchase.
You are buying a dream. But if your perception of risk makes you fear that it could turn into a nightmare, you are unlikely to commit.
This represents a personal loss, because anticipation is an important element of a trip. For the travel industry, though, reluctance to commit represents a major commercial threat. Airlines, hotels and tour operators deal in a uniquely perishable product. If an airline seat remains empty on a journey, or a room is not slept in, there is no way to recover that lost potential revenue. So they like to start selling a year in advance, and tweak prices to maintain a steady flow of bookings.
The murderous year so far in Istanbul, Brussels, Orlando and Nice will hit earnings for summer 2016, but its impact on next summer will be even more serious. The two nations which spend more on foreign holidays than any others in the West are the US and Germany. Americans' sensitivities about overseas travel has been demonstrated in the response to events from the 1990s civil war in Yugoslavia to the 2015 attacks in Paris. Shocks abroad cause a shift to vacations at home, even though this behaviour conveniently ignores the high US death rates from both road deaths and firearms.
Germany is now following the same trend. Each January a social trends foundation - funded, oddly, by British American Tobacco - asks a representative sample of 4 000 Germans about their travel intentions for the year ahead. In 2016, for the first time in a decade, one in five said they had no holiday planned yet.
When asked why, the main issue cited was security, which they rated as more important than weather or value for money.
This year one in three Germans will take their main holiday at home - a proportion not seen for decades.
For the UK, both trends represent bad news: Americans spend much more on vacations in Britain than any other nationality, and the Germans are also crucial for the tourist industry - particularly in more remote areas of the UK.
For France, the impression that the authorities are unable to identify and control individuals intent on mass murder will trigger a sharp downturn in everything from short breaks in Paris or Nice to month-long holidays beside the beach or in the countryside. And for Turkey, 2016 has just gone from bad (forward bookings around 50 per cent down on last year) to very much worse. As crisis grips the country, foreign travel firms will be seeking to extract themselves from 2017 commitments.
While the Turkish economy slumps and hundreds of thousands of tourism workers find themselves unemployed, all we can do is hope that wisdom will prevail and that the government will seek consensus rather than further confrontation. But whatever your attitude to risk, don't bet on it.