U.K. law firm heads believe they have a new drawing card for law graduates. No longer able to compete with U.S. firms on salaries in London, many of the top firms privately say they are attempting to become a popular destination for the best talent through their diversity policies.

Many top U.K. firms already dominate international rankings of LGBT-friendly employers, and they are stepping up their efforts to improve the gender and racial balances in their partnerships. Yet, although it might help firms in the hiring process, the broader purpose in the push for diversity seems to have been lost.

Though great progress may be made regarding race, gender and sexual orientation, law firms are seemingly blind to a variety of diversity issues. There remains a massive bias toward in-shape, privately educated English literature, humanities and law graduates—a disproportionate number of whom went to either the University of Oxford or the University of Cambridge. In the U.K., the age range of lawyers is also limited. Not many partners work past the age of 65 and almost all trainee intakes at top firms are in their 20s. Where are the maths graduates who come from other industries and requalify as lawyers? Where are all the pensioners who decide to work into their 70s? When was the last time you met a lawyer with disabilities?

It is relatively easy to focus on specific quotas, but diversity is actually a far broader topic than race, gender and sexual orientation, and tackling those three areas should not be seen as a panacea to the problem.

Perhaps as they approach the issue of diversity, firms should think about what they are trying to achieve. Impressing graduates because you are diverse is obviously a benefit, but few would say it is the primary motivation for such efforts. In theory, at least, a bigger reason is surely to avoid groupthink. Employing a diverse range of people ensures creativity and helps guard against everyone remaining oblivious to certain problems.

Although partnerships with more women, ethnic minorities and LGBT lawyers are a good thing, I've never heard it said that they lead to more healthy debate on important decisions. In fact, they may all still think the same way because they were hired to fit in with a firm's culture. For decision-making purposes, some experts in the area suggest that diversity of skill, knowledge and opinion, rather than diversity of readily observable demographics, is key. In other words, law firms need people who not only look different, but think different.

This may require firms to bring in more old people, or those who have spent time in a different industry, or even those who are not as well educated in the traditional sense. But perhaps it simply requires them to seek out people who have a different mindset than what they are used to. That sort of diversity almost certainly leads to debate and healthy disagreement.

Fixated as firms are on the idea of a collegiate culture, encouraging dissent, especially below the partner level, is tantamount to heresy. But for firms keen on innovation, this is a necessary part of the creative process. I know of one person who works at a top British company famed for its innovation who says the institution actively hires people who question decisions, hoping to encourage debate and disagreement. It might cause internal friction, but it seems to help the company produce top-quality inventions.

Selling yourself as a firm that welcomes dissenting opinions might seem counterintuitive to graduate recruitment teams. But for law firms serious about developing diversity of thought, it could make all the difference.