Inequality of bargaining power

Inequality of bargaining power in law, economics and social sciences refers to a situation where one party to a bargain (bargaining power), contract or agreement, has more and better alternatives than the other party. This results in one party having greater power than the other to choose not to take the deal and makes it more likely that this party will gain more favourable terms and grant them more negotiating power (as they are in a better position to reject the deal). Inequality of bargaining power is generally thought to undermine the freedom of contract, resulting in a disproportionate level of freedom between parties, and that it represents a place at which markets fail. 
Where bargaining power is persistently unequal, the concept of inequality of bargaining power serves as a justification for the implication of mandatory terms into contracts by law, or the non-enforcement of a contract by the courts.
Historical development.
The concept of inequality of bargaining power was long recognised, particularly with regard to workers. In the "Wealth of Nations" Adam Smith wrote,
Beatrice Webb and Sidney Webb in their treatise "Industrial Democracy" significantly expanded on the critique of 19th century labour conditions and advocated a comprehensive system of labour law contained a chapter called, "The Higgling of the Market". They argued that the labour market was dominated by employers, and therefore had the same effect as monopsony. Workers generally are more under pressure to sell their labour than an employer is under to buy it. An employer can hold out longer, because typically he will have greater financial reserves. This means that much labour is supplied merely out of necessity, than free choice (shifting the supply curve to the right) and it is a false kind of competitive environment. The Webbs also pointed out that discrimination can decrease job opportunities for women or minorities, and that the legal institutions underpinning the market were skewed in favour of employers. Most importantly, they believed that a large pool of unemployed people was a constant downward drag on the ability of workers to bargain for better conditions.
The Webbs felt that these factors all added up to systemic inequality of bargaining power between workers and employers. The first ever use of the phrase "inequality of bargaining power", however, appears to have been by the British philosopher, John Beattie Crozier in "The Wheel of Wealth".
"The real measure of market power is not whether a supplier presents his terms on a take-it-or-leave basis but whether the consumer, if he decides to ‘leave it’ has available to him a workably competitive range of alternative sources of supply. Whether this is or is not so simply cannot be derived intuitively from the fact that a particular supplier is offering non-negotiable standard-form terms. It is a matter for independent inquiry. If the market is workably competitive, any supplier offering uncompetitive standard form terms will have to reformulate his total package of price and non-price terms to prevent consumers (at least consumers at the margin, which are the decisive consideration in such a market) from switching their business to other competitors...
Non-economists often overlook the importance of marginal analysis in this context. For example, if only 10 per cent of the buyers of insurance policies or dry-cleaning services studied all terms scrupulously before contracting and were influenced in their choice of policy by their evaluation of the so-called fine print clauses, and if no supplier of insurance or dry-cleaning services was able to ‘term discriminate’ between these consumers and other consumers in the market, there would be strong competitive pressures on each supplier to adjust the terms of his contracts so as to avoid losing this potential business...
When one asks why, many consumers probably rely in part on the constraints imposed by other consumers at the margin (ie, they let the market shop for them)."
"The point is obvious but worth making because it affects the conditions under which relief should be given: whereas advice as to value will normally save the contract with the ‘poor and ignorant person’, the master of the ship drifting onto the rocks would still have been open to exploitation even if he had had the entire House of Lords on board to advise him."
Legal use.
"Schroeder Music Publishing Co Ltd v Macaulay" [1974] 1 WLR 1308, 1316, per Lord Diplock: “To be in a position to adopt this [‘take it or leave it’] attitude toward a party desirous of entering into a contract to obtain goods or services provides a classic instance of superior bargaining power.”
Consumer laws.
"In so far as the reduction of costs of production and distribution thus achieved is reflected in reduced prices, society as a whole ultimately benefits from the use of standard contracts… The use of contracts has, however, another aspect which has become increasingly important. Standard contracts are typically used by enterprises with strong bargaining power. The weaker party, in need of the goods or services, is frequently not in a position to shop around for better terms, either because the author of the standard contract has a monopoly (natural or artificial) or because all competitors use the same clauses. His contractual intention is but a subjection more or less voluntary to terms dictated by the stronger party, terms whose consequences are often understood only in a vague way, if at all."
Labour laws.
"The legislature has also recognized the fact, which the experience of legislators in many states has corroborated, that the proprietors of these establishments and their operatives do not stand upon an equality, and that their interests are, to a certain extent, conflicting. The former naturally desire to obtain as much labor as possible from their employees, while the latter are often induced by the fear of discharge to conform to regulations which their judgment, fairly exercised, would pronounce to be detrimental to their health or strength. In other words, the proprietors lay down the rules, and the laborers are practically constrained to obey them. In such cases self-interest is often an unsafe guide, and the legislature may properly interpose its authority."
"The inequality of bargaining power between employees who do not possess full freedom of association or actual liberty of contract and employers who are organized in the corporate or other forms of ownership association substantially burdens and affects the flow of commerce, and tends to aggravate recurrent business depressions, by depressing wage rates and the purchasing power of wage earners in industry and by preventing the stabilization of competitive wage rates and working conditions within and between industries."
"It is easy now to see that Parliament in 1906 might have felt that the only way of giving labour an equality of bargaining power with capital was to give it special immunities which the common law did not permit. Even now, when the scales have been redressed, it is easy to see that Parliament might think that a strike, whether reprehensible or not, ought not to be made a ground for litigation and that industrial peace should be sought by other means."