Advertising and publicity has lead us to believe that loads of rich, creamy lather is a good thing in a product. But is that really true? It's definitely a lovely feeling, spreading those tiny bubbles over your hair and body, but you might not be doing much to cleanse if you've got excessive amounts of lather.
Lather is actually the reaction between a surfactant (cleansing ingredient), water and air. The reaction between a surfactant, water and oil (and thus dirt / grease) won't actually create a foam or lather in the same way. So with that in mind, you could say that lather is nothing more than an indication of over-use...
Shampoo is probably the product that has gained (and suffered) the most from this advertising myth. Lather really has no use except as a tactile addition to a product - once it had reacted with air the product doesn't really do anything to the hair. However, most of us would find it odd to shampoo our hair without loads of bubbles, and lather does help us to asses where we need to add more shampoo.
Often when people switch to natural based or high-end shampoos, they are surprised at how little lather they get. Surely a pricey product should 'work' just as well as a cheaper one?
Well, cheaper products are often loaded with cheap surfactants (like the infamous Sodium Lauryl Sulphate) combined with yet more foaming agents, to give you that comforting fluffy lather. More expensive offerings will, in most cases, rely on more useful ingredients that benefit your hair in different ways. With all these crammed in, there's less room for stuff that creates lather, meaning when you wash your hair you may find you get less lather. This doesn't necessarily mean your hair isn't clean, just that the product has been more successful in doing what it was supposed to do.
The girls at Beaut.ie wrote a great piece on Kerastase products which is a good example of this issue. A lot of people over-use their products because they don't lather like rival brands. It's easy to think if a shampoo doesn't produce a thick lather that it's not working, but this really is a myth. A bit of initial lather (try adding a drop of water to your hand when you've squirted your shampoo into it if you don't already) will help you to spread the product evenly, but don't get scared if you don't have huge handfuls of foam with your first wash. If you do, you're using too much.
If you're still finding this hard to believe, think of it this way: Lather is the product's way of showing you you're using too much. Brands want you to buy their products more often, so they start to promote the idea that lather is a good thing. You develop a liking for lather, and use more product as a result. You have to replace your bottle of shampoo / shower gel more often, and the brands succeed in making loads of money off a myth.
More info: The RSC on Surfactants | Hair2Stay Shampoo pdf