Here is an excerpt from an article written by Claudio Fernández-Aráoz for Harvard Business Review and the HBR Blog Network. To read the complete article, check out the wealth of free resources, obtain subscription information, and receive HBR email alerts, please click here.

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Over the course of my career, I’ve spent countless hours talking to and hearing from leaders around the world. I’ve interviewed thousands of candidates for managerial roles and tracked the performance of those I successfully hired. I led the global management appraisal practice of our own executive search firm, Egon Zehnder. And I’ve spent years with colleagues at Harvard Business School and other academic institutions researching what makes people effective in their jobs.

One key lesson I’ve drawn from all this experience? The most successful leaders are the ones who continue to learn and grow, and the best way to help yourself – or your team — do that is through assignments that involve increasing complexity.

Several years ago, I worked with Ken Aramaki in Egon Zehnder’s Tokyo office to compare the potential of senior Japanese executives (that is, objective assessments from our consultants about their ability to take on bigger roles and responsibilities) and their competence (that is, objective assessments of their strategic orientation, market insight, customer impact, results orientation, leadership, ability to collaborate and influence, etc.) against the average scores for those metrics from all the executives in our worldwide database. What we found was an incredible paradox. Japanese professionals had higher potential than the global average but lower competence.

Japan’s educational institutions and cultural work ethic give its managers a jump-start in their careers, but most companies don’t continue the development process as far as it could go. A typical leader rises through the ranks of one division, in one company, waiting respectfully for promotions that usually come only when he’s the most senior person, with the longest tenure, in line for the spot.

Recently, I became aware of the case of a Tokyo-based global conglomerate with a highly diversified business portfolio that could not find even one qualified CEO successor. This company, which stretched into all sorts of industries and markets, offering numerous strategic challenges, should have been an ideal training ground for executives. However, only one of its senior managers had worked in more than a single business line. The top 12 leaders had spent an average of just one year working outside of Japan. And their English language skills were quite limited. In sum, none were suitable candidates to succeed the CEO. The sad part is that all of them had started off strong: they were engineers with an average tenure of more than 20 years in R&D and product strategy and marketing. But that potential had been squandered.

The fact is that giving people bigger jobs with fancier titles and larger salaries won’t make them better. More complex assignments will.

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Here is a direct link to the complete article.

Claudio Fernández-Aráoz is a senior adviser at the global executive search firm Egon Zehnder and the author of It’s Not the How or the What but the Who (Harvard Business Review Press, 2014).