Today's typical grocery store shopping cart is much larger today the shopping carts of yesteryear. The question is: why?

A behavioural economist would observe that people buy more when shopping carts are larger. For example, an AER article by Wansink, Just and Payne notes:

….consumption can also be unknowingly influenced by environmental cues—benchmarks
or reference points—that may subtly suggest a
consumption norm that is appropriate, typical,
reasonable, and normal. ….large packages, plates, serving bowls, and even pantries have all been
shown to increase how much a person serves
and consumes by 15 to 45 percent

If grocery stores figure that larger shopping carts translate into higher sales volumes, one might expect them to increase cart sizes. 

Eaton, Eaton and Allen take issue with this explanation in the 2009 edition of their Microeconomics textbook:

If you're thinking that store owners just increased the size of carts to fool customers into making larger purchases, then you are off track. Customers might get fooled once, but eventually they would learn that larger carts are bad things, and would only visit stores with small carts. As a budding economist, you want to avoid lines of reasoning that suggest people habitually do things that make them worse off… (p. 64)

The problem with this argument is that it flies in the face of the abundant empirical evidence that people habitually overeat, overspend, and do other things that make them worse off.  

Yet there is a good point here: an economist should start with the premise that people aren't stupid. So is there an alternative explanation? Eaton, Eaton and Allen say there is:

One of the major changes to have taken place in the past 50 years has been the continued entrance of women into the workforce. When women were more likely to be at home they had more time on their hands and had schedules that were flexible. Multiple trips to a grocery store had the benefits of fresher produce and reduced required storage space at home in terms of pantries and freezers. However, the cost was that it involved large amounts of time. As women continued to work outside the home, their time became more scarce and a substitution took place: fewer but larger trips to the local grocery stores. Larger shopping carts are not a marketing tool of exploitation, but just another example of substitution taking place. 

There is one sensible observation here: today, people buy in bulk, buying more when they visit the grocery store. The question is: why?

If the trend towards larger trips to the grocery store was caused by increased female labour force participation, I would expect to see big stores and big shopping carts predominating in areas where female labour force participation is high, and more traditional, small shopping cart shopping experiences in areas where female labour force participation is low. I would expect to see relatively few at-home parent pushing mega-carts around mega-stores. That's simply not the case.

Why not? Families with one income, all else being equal, have less cash than families with two incomes, hence are relatively sensitive to price. Groceries often cost less when purchased in bulk. Mega-stores have lower costs due to economies of scale, low-rent locations, and monopsony power that allows them to extract price concessions from suppliers. Some of these cost savings are passsed onto consumers in the form of lower prices. Moreover, bulk sales facilitate price discrimination – a store can charge the last minute shopper $1.79 for the can of tomatoes he needs tonight, and charge the price sensitive planner $12 for 12 cans of tomatoes. The family with a full-time homemaker shops in bulk to take advantage of price discrimination that works in their favour. 

Large shopping trips also require a concentrated block of time. How much time does it take to go to Costco on a Saturday? Two hours? Three hours? The family with the at-home parent has time to take long shopping trips, and the flexibility to visit Costco when it's not insanely busy. The time-strapped jobs+kids parents are the ones shopping for bread for tomorrow's sandwiches at 9:45 at night at the local small-cart supermarket.

What about the argument that, 50 years ago, people shopped daily to buy fresh produce? That was then, this is now. People have larger houses and better refrigerators. Food storage and transportation technology has improved. Apples, potatoes, and onions are harvested in August and September, then stored and sold year round. What can't be grown locally and stored is trucked or shipped in from southern climes – tomatoes from Mexico, grapes from Chile, asparagus from Peru. Avocados, bananas and pears are sold while still hard, so the customer can ripen them at home. For most produce, it doesn't matter whether a person shops daily or weekly – regardless of their labour market status. 

Truly fresh vegetables are now a niche market served by specialist stores. Yes, they do have relatively small shopping carts. Interestingly, my local fruit-and-vegetable store, Farm Boy, has a large and extensive deli counter, selling ready-made meals. This suggests that their target audience includes – without necessarily being limited to – people who work outside the home.

So what has caused shopping carts to grow? My preferred explanation is simple: people have higher incomes now than they once did. When people have higher incomes, they buy more stuff, hence need larger carts to put it all in. Fifty years ago, Pampers had only just come on the market. Thrifty housewives used rags instead of paper towels, mops instead of Swiffers, and ground coffee instead of Tassimo pods. Yummy single-serving yoghurts and cheese strings had not been invented yet, and bottled water was this strange European thing. A well-stocked refrigerator had one type of mustard, not four; a well-stocked cupboard had one or two types of vinegar, not six (white, red wine, white wine, cider, malt and balsamic). With all of this extra stuff, people need bigger carts.

It could be argued that Eaton, Eaton and Allen are right about the causal impact of female labour force participation on shopping cart size, just wrong about the mechanism – women entering the workforce prompted an explosion of convenience foods and labour saving goods, and this has caused the growth in shopping carts. That's an old story, but one that has been called into question. For example, Greenwood, Seshadri and Yorukoglu, in their paper Engines of Liberation, argue that it is not women entering the labour force that caused the revolution in household technology. Instead, the development of washing machines, refrigerators, microwaves and other household appliances made it possible for women to work outside the home and still cope with the demands of family life. Mega-stores and mega-shopping carts are part of that technological change – but which way the causality runs is hard to determine.

There is just one last possibility to consider. The shopping cart was only invented in 1937, and took some time to be widely adopted. The shopping carts of 50 years ago were still relatively new models. It's entirely possible that they were simply badly designed, and too small for the average shopper even then. 

So here's the question: what's the best explanation for the growth of shopping carts:

1. Behavioural economics: people buy more when shopping carts are larger
2. Rising female labour force participation causing people to take fewer but larger shopping trips
3. Rising incomes mean people buy more stuff hence need larger carts
4. The original carts were badly designed in the first place

Update: in response to the  comments, here is an additional explanation:

5. Supermarkets are larger today than in the past, for reasons not captured in (1) to (4) above, such as the introduction of universal bar codes and scanners. Larger supermarkets require bigger carts.