According to a report completed by Calvert Investments just a few months ago, most companies in the S&P 100 are improving their diversity initiatives and reporting, while a handful are falling far behind the pack. Workplace diversity may not be one of the first things you think of when considering investing in a company, but for a company to be truly great it needs the perspective of many individuals.

The companies in the study were ranked on 10 different diversity criteria, which included internal and external diversity initiatives, board diversity, diversity of their highest-paid executives, Equal Employment Opportunity (EEO) policies, and director selection criteria, as well as the company's overall commitment to diversity.

In its study, Calvert defined diversity as: "Nondiscrimination and equal opportunity concerning recruitment, hiring, pay, promotion, training, and tenure without regard to race, gender, age, religion, national origin, ethnicity, sexual orientation, gender identity and expression, HIV/AIDS status, medical status, and mental and physical ability."

Using its definition and looking at public data, as well as reaching out to companies to clarify their diversity data, Calvert found the following companies were the least diverse out of all the companies in the S&P 100:

1. National Oilwell Varco (NYSE:NOV) -- Diversity score of 15: This oil and gas component and equipment company received the third-lowest rating in the report, partly because it was one of only two companies in the study that doesn't have any diversity on its board of directors. In addition, National Oilwell Varco has no women or minorities in the C-suite, as a Forbes overview on the report noted earlier this year. . When it comes to diversity, oil and gas companies typically tend to fall behind other industries.

According to Diversity Inc., which compiles a list of the top 50 most diverse companies each year, no oil company has made it on its list since 2004. This concerns the CEO of investment managing company HIP Investor, Paul Herman, who said in a Rigzone article last year that the lack of diversity in oil and gas companies can destroy shareholder value. 

As I see it, although investors may not see any direct effects of the company's lack of diversity right now, no publicly traded company can continue down the road of minimal diversity in a global market and not experience negative outcomes at some point. It's in the best interest of National Oilwell Varco, and its investors, to address its diversity issues now so it can make changes to its policies in the near future.

2. Simon Property Group (NYSE:SPG) -- Diversity score of 10: Simon Property is one of the largest real estate companies in the world and was mentioned in the report as an example of a company that treats diversity as a "compliance matter." The study said that Simon Property's diversity policy is treated as a human resources matter and the company isn't as public with its diversity programs or data as others are. Calvert said that companies in this grouping typically don't have strong diversity leadership.

Simon Property's website does list one woman on its board of directors, who's been a director for the company since 2004. The company also announced last month -- after the Calvert report was released -- that Liz Zale, formerly the vice president of investor relations for Nielsen Holdings, is a new senior vice president with Simon.

3. Berkshire Hathaway (NYSE:BRK.B) -- Diversity score of 5: One of the reasons Warren Buffett's company, favored by many investors, received the lowest diversity score out of all the S&P 100 companies was because it was the only company that specifically stated it does not consider diversity in selecting its board of directors, according to the report.

Calvert quoted Berkshire Hathaway as saying, "Berkshire does not have a policy regarding the consideration of diversity in identifying nominees for director. In identifying director nominees, the Governance Committee does not seek diversity, however defined."

Despite this, Berkshire just added a third woman to its board of directors a few days ago, and just last week Buffett wrote a Fortune article entitled, "Warren Buffet Is Bullish... on Women." In it he says, "The closer that America comes to fully employing the talents of all its citizens, the greater its output of goods and services will be." But just as with National Oilwell, Forbes noted that Berkshire doesn't have any women or minorities in the C-suite.

Investing in diversity
Diversity obviously isn't the only aspect that should be considered when looking to invest in a company, but it shouldn't be overlooked. Companies are built on good ideas and hard work, which find their source in different perspectives and viewpoints. To truly invest in great companies, long-term investors should consider the benefits of a diversified company workforce, and make it part of their investing equation.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.