The business case for diversity seems intuitive. Teams of mixed gender, ethnicity, physical ability, age and sexual orientation are more representative of customers. They offer a variety of viewpoints and a wider range of experience, which improves decision-making and problem-solving. Most business leaders seem to agree with this assertion.

Research by recruiter Korn/Ferry in November 2013 found that almost all the global executives it polled believe diversity and inclusion can boost results.

But what evidence is there that they are right?

First, a little debunking is needed. The research most often cited as evidence is provided by separate studies by consultants Catalyst and McKinsey published in 2007. Both compared the financial performance of large organisations according to the gender diversity at senior levels, and both found that high returns on equity correlated with greater diversity.

Rather than irrefutable proof, however, neither study was able to show a causation.

Research last year by the New York-based Center for Talent Innovation (CTI), however, began to take us a step closer.

Involving more than 40 case studies and 1,800 employee surveys, it looked at what it termed “two-dimensional diversity”, namely “inherent diversity”– such as gender and race – combined with “acquired diversity” – such as global experience and language skills.

It found that publicly traded companies with two-dimensional diversity were 45 per cent more likely than those without to have expanded market share in the past year and 70 per cent more likely to have captured a new market. When teams had one or more members who represented a target end-user, the entire team was as much as 158 per cent more likely to understand that target end-user and innovate accordingly.

A 2012 research report from Deloitte, “Waiter, is that inclusion in my soup?”, edges us further towards causation. It is based on the experiences of 1,550 employees in three large Australian businesses. It identified an 80 per cent improvement in business performance when levels of diversity and inclusion were high.

An American Sociological Association study supports this, finding that for every 1 per cent rise in the rate of gender diversity and ethnic diversity in a workforce there is a 3 and 9 per cent rise in sales revenue, respectively.

If this seems far-fetched, consider it from a company’s perspective. Jyoti Chopra, global head of diversity and inclusion for BNY Mellon, the bank, says: “We have offices in more than 35 countries, across 100 markets, and a workforce of around 50,000 people. Our employees have to be able to work effectively in cross-border teams, they’ve got to be able to deliver goods and services . . . for clients ranging from individual investors to corporate institutional clients. Diversity is an imperative.”

Capital markets and investors now link this to corporate performance. Ms Chopra talks of clients “proactively looking for diverse engagement teams” and “increasing demands for diversity data and information in retirement saving plans”.

Evidence of internal organisational benefits is also hard to ignore. Researchers Horwitz and Horwitz reviewed 20 years of research on team diversity in 2007 and identified a positive relationship between diversity and team performance.

An experiment by Massachusetts-based Tufts University demonstrated that diverse groups perform better than homogeneous teams by when it deployed 200 people in mock juries – the mixed juries all performed better than those comprising only white or only black jurors. Groupthink may lead to a cohesive team, but one that will happily agree on the same costly mistake.

The CTI study found that, “ideas from women, people of colour, LGBTs, and Generation Ys are less likely to win the endorsement they need to go forward, because 56 per cent of leaders don’t value ideas they don’t personally see a need for . . . the data strongly suggest that homogeneity stifles innovation.”

Stephen Frost, head of diversity and inclusion for the London Olympics and now for KMPG, writes in his book The Inclusion Imperative that discrimination against women, homosexuals and disabled people is estimated to cost $64bn a year in the US alone. In addition, writes Mr Frost: “When gay people remain in the closet, they are 10 per cent less productive than when they feel able to be themselves.” One recent seminar advocated a 30 per cent productivity increase. Yet 41 per cent of American LGBT workers remain closeted at work.

At BNY Mellon, Ms Chopra believes a diversity strategy is also integral to keeping staff. Female retention rates at the bank are now higher than male. More than 20 per cent of BNY Mellon’s global workforce are members of an internal minority employee network, up from 16 per cent a year ago.

Finally, if an organisation stubbornly refuses to change its predominantly straight-white-male ways, then it will increasingly find it hard to recruit.

According to McKinsey, if the employment rate for women remains constant, Europe can expect a shortfall of 24m people in the active workforce by 2040. If the rate can be raised to the same level as for men, then the projected shortfall drops to 3m.

Guelabatin Sun, Deutsche Bank’s global head of diversity, notes that the global talent pool is changing. “If we want to continue to attract the best talent, we need to be reflective of the talent in the specific market and offer a work environment that employees want to be a part of,” she says.

There is a twist in this evidential tale, however. Almost all the research on workplace diversity is unanimous on one thing: it can go wrong. Organisations without proper managerial or cultural understanding of diversity can end up with heightened conflict and reduced productivity.

As Deloitte’s “Only skin deep?” 2011 report says, “it is not enough to create a corporate version of Noah’s Ark bringing in ‘two of each kind’ . . . There is a clear argument for actively managing diversity rather than assuming we will naturally derive the benefits”.

A McKinsey 2012 report found that, “though CEOs made gender diversity a priority in more than 80 per cent of our 60 participating companies, only about half of employees surveyed from the same companies agreed that the CEO is committed to it”. The business case for diversity may now be proven, but it seems that action is lagging behind words.

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Minority markets: Women and LGBT consumers reward company diversity

Businesses without a varied mix of employees can miss out on the spending power of minority markets.

“Women Matter”, a 2007 report by McKinsey, the consultancy, found that women were the driving force behind 70 per cent of household purchases in Europe and influence 60 per cent of car purchases in Japan.

Thus an all-male organisation selling household consumer goods risks being less in touch with its market than one boasting gender parity.

Some minority consumer groups also wield their influence proactively. The so-called “pink pound” refers to the influence of lesbian, gay, bisexual and transgender (LGBT) consumers.

In 2009, Witeck, the US communications company and Harris Interactive, the market researcher, found the estimated spending power of the LGBT community to be £81bn in the UK and $712bn in the US.

It also suggested that 78 per cent of the LGBT community, their friends and relatives would switch brands to companies known to be LGBT-friendly.

With annual listings of top gay-friendly employers becoming more prevalent, it takes only a quick internet search to see which companies are well positioned to capitalise on diversity and which remain firmly in the closet.