All investors hope their stock returns will be sustainable, at least in the respect that they'll keep on making money over the long haul. However, more investors should contemplate the growing realization that investments in sustainable companies outperform the market by a long shot.

High sustainability = high performance
Researchers at Harvard Business School and London Business School recently released a study showing that a portfolio of 90 "high sustainability" organizations significantly outperformed 90 "low sustainability" peers over the long term. This outperformance includes stock performance and accounting performance.

"The Impact of a Corporate Culture of Sustainability on Corporate Behavior and Performance" study didn't define "long term" as a mere couple of years, either. It tracked the two groups' financial performance over an 18-year period. The high sustainability companies were defined as those that had implemented a significant number of environmental and social policies since the early to mid-1990s.

According to the study, a $1 investment in a value-weighted portfolio of high sustainability firms in 1993 would have burgeoned to $22.60 by the end of 2010. The same investment in a portfolio of low sustainability firms would have only grown to $15.40 in that same time frame.

The authors also found that high sustainability firms significantly outperformed low sustainability companies on metrics including return on assets and return on equity metrics.

Furthermore, an equal-weighted portfolio of high sustainability firms also outperformed a portfolio of traditional companies.

Sustainability before sustainability was cool
The authors acknowledge that widespread corporate sustainability pushes have only reached mainstream recently, although some companies have been quietly implementing such long-term policies for nearly two decades now.

Consider corporations that have forged ahead of the curve in connecting incentives to environmental factors. For example, in the mid-1990s, Intel (Nasdaq: INTC) began linking executive compensation to environmental metrics, and in 2008, it connected all employee bonuses to environmental goals.

High sustainability firms are also defined as those that take many stakeholders into account instead of focusing solely on shareholder returns. Timberland (recently acquired by VF (NYSE: VFC)) has multiple plans in place for stakeholder engagement, including social media platforms, employee and customer surveys, and initiatives like its nutritional label and green index.

The report points out a very real reason why such companies outperform, and it's common sense. The reason may be that "they are able to attract better human capital, establish more reliable supply chains, avoid conflicts and costly controversies with nearby communities (i.e., maintain their license to operate), and engage in more product and process innovations."

The authors called out Germany's Siemens (NYSE: SI) as an example, generating 20 billion euros in sales from its environmental portfolio alone.

Patience is a virtue ... and lucrative, too
Although the report didn't reveal the 90 "high sustainability" companies to admire (nor did it reveal the 90 "low sustainability" companies to revile), there are plenty of companies that emphasize sustainability and understand that recognizing the long-term interests of many stakeholders creates the best long-term businesses.

Whole Foods Market (Nasdaq: WFM) and Starbucks (Nasdaq: SBUX) are easy examples; both provide health benefits to employees, embrace important elements like fair trade, and give back to communities both here and abroad. These companies understand the holistic relationship between customer, employee, community, environment, and shareholder.

And what about Google (Nasdaq: GOOG)? When it went public in 2004, its founders' IPO letter made it clear that "shareholder value" was hardly its only goal:

We aspire to make Google an institution that makes the world a better place. In pursuing this goal, we will always be mindful of our responsibilities to our shareholders, employees, customers and business partners. ... And now, we are in the process of establishing the Google Foundation. We intend to contribute significant resources to the foundation, including employee time and approximately 1% of Google's equity and profits in some form. We hope someday this institution may eclipse Google itself in terms of overall world impact by ambitiously applying innovation and significant resources to the largest of the world's problems.

Personally, I believe more investors should view the "sustainability" of their stock portfolios through a far wider lens than the "sustainability" of stock price appreciation. Investing in truly good companies is the smartest thing true long-term buy-and-hold investors can do.

After all, as the study's authors note: "the results suggest that this outperformance occurs only in the long term. Managers that are hoping to gain a competitive advantage in the short term are unlikely to succeed by embedding sustainability in the organization's strategy. Similarly, investors in High Sustainability firms must be patient."

The best way to build a strong portfolio for the long haul is to target the companies that are building the most solid foundations by doing good in the world. Clearly, the future dividends are comprised of real returns on investment as well as emotional ones.

Check back at Fool.com every Wednesday and Friday for Alyce Lomax's columns on environmental, social, and governance issues.