There is nothing "contrived" about waterbed effects ('Sainsbury's says waterbed effect theory is 'contrived'," The Grocer, 2 June, p13). They are both a theoretical and practical possibility. Waterbed effects can be expected in markets where a buying advantage feeds into a selling advantage and vice versa. This can drive virtuous circles of growth for chains with such advantages, and vicious circles of decline for the less fortunate. Also, anti-competitive buying and selling practices by large chains can accelerate these circles. In such circumstances, it is no surprise that large retail chains grow at the expense of small ones. The Competition Commission has yet to finish its analysis of supplier terms in the present groceries market inquiry. But in its previous monopoly inquiry in 2000, the Commission demonstrated that small grocery chains could face paying 10% or more buying goods compared with leading chains, with a strong correlation between market share and purchasing effectiveness. Any retailer faced with such a supply price disadvantage is at a competitive disadvantage in selling to consumers and will struggle to maintain its market position. Any resulting loss of market share will place it at a further buying disadvantage, making it even harder to compete . The reverse applies for a retailer growing its market share, extending its buying advantage etc. Yet, the more the competitive playing field is tilted, the more likely it is that competition will be reduced - and the consumer made worse off as a result .