As companies age and become more successful, they grow.  As they grow, company leaders have more to lose, so they may be more cautious.  And, Steve Coley, a director emeritus for McKinsey & Company and co-author of The Alchemy of Growth, says that usually leads to declining growth "as innovation gives way to inertia:"  

In order to achieve consistent levels of growth throughout their corporate lifetimes, companies must attend to existing businesses while still considering areas they can grow in the future. The three horizons framework—featured in The Alchemy of Growth,—provides a structure for companies to assess potential opportunities for growth without neglecting performance in the present.

Horizon one represents those core businesses most readily identified with the company name and those that provide the greatest profits and cash flow. Here the focus is on improving performance to maximize the remaining value. Horizon two encompasses emerging opportunities, including rising entrepreneurial ventures likely to generate substantial profits in the future but that could require considerable investment. Horizon three contains ideas for profitable growth down the road—for instance, small ventures such as research projects, pilot programs, or minority stakes in new businesses.

Coley explains the three horizons of growth further in an interactive piece over at McKinsey Quarterly.  Click here to watch and listen.