At least that was the gist of an idea that was being discussed on the Counterpoint podcast this week. A study of shopping habits was undertaken in Holland, where huge numbers of people walk or cycle to the shops. They tend to shop much more often than car-borne shoppers, but buy fewer items on each occasion.
If all you have is two panniers on your bike for carrying the shopping home, you won't buy more than will fit into the panniers. If you have a car with a cavernous boot, the temptation is there is buy lots of unnecessary impulse purchases since there is no "dead arm" penalty for carrying them home.
I walk to the shops most of the time, and the "dead arm" penalty is what I get about 3/4 of the way home when I have 10 kilos of shopping bags hanging from each hand - the numbness starts in the fingers and slowly works its way up the arm, until one has to stop and take a breather.
Anyway, impulsiveness is a function of transportation option. People also tend to be more impulsive if they are shopping with someone else.
The guy being interviewed made a good point that studies that talk about impulse shopping always inflate the percentage of impulse purchases - mainly because quite a few of them are funded by the Point of Purchase Institute (I think that is what it was called), and they have a vested interest in promoting the idea of people impulse buying chocolate or chewing gum or magazines at the checkout. His studies had shown that the number of people impulse buying was much lower than is generally thought.
There is also a difference between "category impulse shopping" and non-category impulse shopping - I am not sure if I have the terms right. Category impulse shopping is where you go to the shops with the aim of buying cereal, but you don't choose what variety to buy until you are standing in the cereal aisle. You know you want to buy something from that category, but you don't know specifically what you will buy.
Non-category impulse shopping is where you buy something totally out of the blue - like a set of textas.
Category impulse shopping is fairly common, but non-category impulse shopping is not. To inflate the numbers, marketers will lump the two together and proclaim that we are a nation of impulse shoppers, when really, we aren't.
All I can advise is this - write up a shopping list before you go.