As we're going into the Friday after Thanksgiving and Black Monday, coming up soon (I'll be in Paris on Black Monday, so as far as I'm concerned, I hope not to be impulsively clicking on anything) I'm reminded of a why we buy on impulse, vs. a considered purchase (the Hunt, so I'm told) which is dealt with in a post titled The Whys of Buy: Impulse Buying from Out of My Gord which shared Jared Spool's results from a study of impulse buying.

In this study, 30 people, who had real things they wanted to buy were given some money and sent them on an online shopping trip to a few preselected sites that had what they were looking for.  Results?

"…While many of the participants bought what they were looking for, a significant number, 34%, also added other items into their shopping cart that weren’t on their original lists. Was it because the prices were irrestible? No, in fact, only 8% of the impulse purchases were because of price."

"…Most of the impulse items were chosen while browsing through the category pages. They had chosen a category based on what they were shopping for and had found related items that struck their interest and were subsequently added to their cart."

An earlier study was cited run by Carnegie Mellon, Stanford and the MIT Sloan School of Management found that impulse buying was a decision of "pleasure" over "pain" and to get people to impulse buy more often - increase the immediate pleasure while decreasing or eliminating the pain associated with the purchase:

"..The results of this paper support an alternative perspective that views consumers as trading off the immediate pleasure of making a purchase against an immediate pain: the pain of forking out the money for the item. The results can explain the growing tendency of consumers to overspend when purchasing items with credit cards instead of cash, because consumers do not immediately pay for items charged to credit cards and the "pain" of the potential loss is minimized."

"…Economic policies designed to promote savings would thus need to take this into account. It also suggests that differences in how much people spend and save may be partly explained by differences in the degree to which they find spending money painful.

Gord Hotchkiss explains impulse buying in more detail:

".. when we anticipate buying something, the pleasure center, the nucleus accumbens, is activated. We begin picturing ourselves in possession of a product and visualizing ourselves using it. We start to build neural pathways that reinforce what it would be like to have the product. But, if the price is excessive, the study found that the brain has a shut off mechanism.  A part of the brain known as the insula is activated and the part of the brain we use to balance gains versus losses, in other words, is the product worth the price, the medial prefrontal cortex, begins shutting down. We literally put the purchase out of our mind because the price is more than we’re willing to pay.

Ok, so here's a "formula" for "impulse buying":

1. Expose the target audience to the "stimuli" (items they're looking for - use search query logs and search engine referrals to determine why people are coming to the site and what they wanted to accomplish.  Then, match up the Stimuli with the searcher who's looking for it (PPC and Behavioral Targeting can both be employed).
2. Figure out what the price point for buying "stimuli" items are for the target audience(s) (that might take some research) and if it's possible to offer financing, or some way to defer having to immediately fork over the money.
3. Offer the item(s) (stimuli) at a lower price than what the target audience thinks it's worth (perception is worth more than reality here) and incentive to enourage an immediate decision (before the the part of the brain we use to balance gains versus losses becomes involved).

Given what has been shared - how would site owners change their sites around to incourage "impulse buying"?  I have a few suggestions:

1. Provide Research Tools on the site that allow extensive searching and comparision of products and services - make these attractive and easy to use.
2. Build in a "reccomendation engine" (similar to Amazon) that suggests if you like "this" you might also like "that" and need these "additional" items.
3. Study when visitors tend to purchase "impulsively" (record the information) and try to tie it back to the stimulas that created that demand.
4. When demands are seasonal, or externally driven, try to anticipate them beforehand, and prepare a series of offers, running them via A/B Testing (Google Website Optimizer, for example) to find out the best prices and the best times to present the stimulus (offer).

So Impulse buying happens when the pleasure of buying something is greater than the pain associated with paying for it - and it seems as if a merchant is in control of both sides of the equation. 

But, of course, the person (audience) needs to want the item/service in the first place - that's why it's important to first present the information or product informally (stimuli) and then allow for anticipation to kick in - and then  - once that happens, you can present an offer that can be acted on as "impulse".