Departures can be simply a function of politics. For example, a CIO hired by an elected official such as a treasurer or comptroller knows he or she has a limited time to act. "Once the term is up, your time is up," Ms. Neri said.
A major reason for departures is the fishbowl existence of a public plan CIO. "The whole environment is more complex, politically charged and polarized," said Mr. Funston.
"It's a demanding position and people wear down," said Daniel Cummings, the Denver-based senior vice president and managing director for EFL Associates, an executive search firm. "Stakeholders have to be managed effectively."
Consultants said there's also continuing financial pressure as plans constantly compete with the private sector for hiring — and retaining — skilled financial professionals, not only for the CIO job but also for other top-level posts within public pension systems.
The departing giant plan CIOs illustrate the tenure range identified by consultants:
Theodore "Ted" Eliopoulos joined CalPERS in 2007 and was named interim CIO in June 2013 to succeed the late Joseph Dear. He was named permanent CIO in September 2014. His departure was announced May 14; Mr. Eliopoulos said he would leave by year-end and consider working in the private sector.
Vicki Fuller, of the $206.9 billion New York State Common Retirement Fund, has held the job for nearly six years. Her departure was announced May 11. She will leave sometime in the summer. She didn't discuss her next move.
Scott Evans of the $194 billion New York City Retirement Systems has served for four years. On May 29, it was announced that he will depart at the end of June, the end of the plan's fiscal year. He didn't comment on his plans.
There are exceptions to the "typical" tenure. Christopher J. Ailman has been the CIO of the $224.8 billion California State Teachers' Retirement System, West Sacramento, since 2000. Thomas Lee has been the executive director and CIO of the $118.9 billion New York State Teachers' Retirement System, Albany, since 2007, after having served one year as deputy CIO.
On the other side of the tenure bell curve is the $34 billion Connecticut Retirement Plans & Trust Funds, Hartford.
On June 1, Laurie Martin was named CIO replacing Sean Crawford, who departed after only 10 days. Denise Nappier, the state treasurer and sole trustee of the pension system, didn't comment on why Mr. Crawford left. Rather than launch another search, she appointed Ms. Martin with the consent of the state Investment Advisory Council.
Ms. Martin had been deputy CIO as well as interim CIO since Deborah Spalding resigned in February 2017. In addition to Ms. Martin and Mr. Crawford, the Connecticut pension system has had three other permanent CIOs — and three other interim CIOs — since 2007.
Succession planning for public plan CIOs is a challenge, consultants said, because most plans lack the resources to copy the private sector's emphasis on grooming successors for top jobs.
"A lot of plans don't have (formal) succession plans because they don't have the bench strength" of large, experienced staffs, said EFL Associates' Mr. Cummings. Most of his firm's searches for public plan CIOs have resulted in external hires because internal candidates lacked sufficient experience, he said.
Even if a pension plan's governing board believes it has a top-notch internal candidate, "you have to post the job and do a search to show there's no favoritism or optics of preferential treatment," said Ms. Neri of Heidrick & Struggles.
"Once you move away from the top 10 in endowments or public plans, the staff levels are pretty lean," added Mr. Wilbanks. "It's hard to create a laddered system of seniority for the next step in promotions."
Mr. Wilbanks said a search is governed by three factors — the amount of time, job benefits and the search criteria. "The more selective you are and the fewer benefits you offer, the longer the search will take," he said, noting public pension plan CIO salaries are usually fixed or limited by law.
Public plans often have "a roster of internal candidates rather than a specific successor," said Mr. Funston. When conducting a search, a governing board "may underestimate how much money is needed" to lure a candidate and "it may have to get legislative approval to reclassify" the job to provide added benefits.
Among some of the largest public plans, succession planning can be quite informal.
The New York State Common Retirement Fund "doesn't have a formal CIO succession policy," said Matthew Sweeney, a spokesman for state Comptroller Thomas DiNapoli, the fund's sole trustee. "But it is structured to ensure continued and unimpeded operations."
CalPERS lacks a CIO-specific plan, but the system "is committed to succession planning at all levels of our organization, including our executive team members," said spokeswoman Megan White. "This commitment is reflected in our strategic plan."