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Decumulation: The Nastiest, Hardest Problem In Finance

By Timeline 22 Jul 2026
57 min read
For Financial Advisers Only

Decumulation: the nastiest, hardest problem in finance?

WB
Warwick Bloore FPFS
Senior Specialist, Advisory Research Centre, Vanguard
JU
Jake Usher
Host, Timeline

Nobel laureate William F. Sharpe once called decumulation the nastiest, hardest problem in finance. In this Adviser 3.0 session, Vanguard's Warwick Bloore explained why, and set out three areas that reward a closer look: how clients spend, the role of secure income, and how drawdown is managed. What follows is a summary of the themes discussed. It is illustrative rather than prescriptive, and none of it constitutes advice.

Watch the full replay above.

Why decumulation is so difficult

The accumulation stage has a clear objective: build the pot. Decumulation is the opposite, and far messier. Warwick framed it as a problem defined by both uncertainty and risk. On the uncertainty side sit longevity, future expenditure, and the balancing of competing goals such as income, legacy and lifetime giving. On the risk side, once earnings stop a client loses much of their power to course correct, sequence of returns risk bites hardest, and later life can bring vulnerability. Wrapped around all of it is the psychological shift from earning and saving to spending down a finite pot.

Longevity is unknown
Neither adviser nor client knows the time horizon, and the range of outcomes is wide.
Spending is a moving target
Many clients struggle to state what they spend today, let alone project it forward.
Course correction is limited
Once income stops, sequence risk and later-life vulnerability are harder to offset.

It is also a topical problem. The FCA's Retirement Income Advice thematic review (March 2024) flagged shortcomings in the sustainability of portfolio withdrawals, the assessment of risk profile and capacity for loss, and advice suitability, alongside firm controls and process. Those first three are exactly the areas the session set out to explore.

"The nastiest, hardest problem in finance."
William F. Sharpe, economist and 1990 Nobel laureate

Longevity is a distribution, not a number

Using ONS life expectancy data at age 65, average life expectancy is around 86 for men and 88 for women. There is a one in ten chance of reaching 96 and 98 respectively. Set against a typical UK retirement age of 64 to 65, that implies planning horizons stretching from roughly 21 years to well over 30. Planning to the average alone risks leaving a client short in the very scenario, a long life, where running out of money hurts most.

Three ingredients, one real lever

Portfolio longevity comes down to three inputs: investment returns, inflation, and spending. An adviser can flex asset allocation, but markets do what they do, and inflation is outside anyone's control. Spending is the ingredient where advisers have genuine influence, and where clients tend to need the most help.

On spending levels, the PLSA Retirement Living Standards (June 2025) offer a useful anchor. The Comfortable standard is around £43,900 for a single person and £60,600 for a couple. For many advised clients that is a starting point for the conversation rather than an aspiration. At the wealthier end, Forbes' tongue-in-cheek Cost of Living Extremely Well index has run at about 5.0% a year over 41 years against US CPI of 2.8%, a reminder that wealthier clients often experience a higher personal rate of inflation than the headline figure suggests.

Spending patterns themselves are debated. The often-cited U-shaped curve, higher spending early in retirement, a dip in the middle, and a rise later if care is needed, is only one possibility. Other data shows a gradual decline. Faced with that uncertainty, advisers tend to err on the side of caution, which is one reason clients can end up leaving a larger legacy than they ever intended.


Spending approaches

Two familiar rules sit at either end of the spectrum, and neither is perfect on its own.

Pound plus inflation
Set an initial amount, then increase it by inflation each year. Real spending is stable and easy to understand. The trade-off is a higher chance of depletion, because it ignores what the portfolio is doing and is exposed to sequence risk. Bill Bengen's 4% rule (1994) is the famous example, and on Vanguard's updated modelling still shows a greater than 90% chance of a 50:50 portfolio lasting 30 years.
Percentage of portfolio
Spend a fixed percentage of the portfolio each year. Mathematically the pot never runs dry and the client shares in growth, but spending becomes highly unstable from one year to the next, which is hard to live on.

Whatever the starting point, the appropriate rate is highly individual. A client with a short horizon and flexible spending can sustain a higher rate, while a cautious investor facing a 40-year horizon can only support a low one. That nuance is part of what the FCA review was getting at: a one-size-fits-all withdrawal rate is hard to justify.

Dynamic spending: the middle ground

Dynamic spending is a hybrid. Spending rises with inflation, but within agreed floors and ceilings that respond to how the portfolio performs. In good years spending can drift up towards the ceiling; in poor years it trims back towards the floor. Where a client can accept modest flexibility, this can extend portfolio longevity while keeping spending reasonably stable. Warwick suggested three things make it work in practice:

1
Pick the right client. It suits those who want to target a higher spending rate and can flex their real spending when asked.
2
Choose the method. Follow a prescribed set of rules, or keep it broad and conversational. A stochastic cash flow plan is often the most practical way to implement it.
3
Prepare and coach. Introduce the concept up front, define the floors and ceilings in advance, and revisit at the annual review so trimming back is never a surprise.

In essence, it is a deal with the client: target a more ambitious spend in return for a willingness to pare back when markets demand it. Signposting that up front makes the conversation far easier when the time comes.


The role of annuities

Economists have long puzzled over why annuity take-up is so low, even when rates have been attractive. The explanation is largely behavioural: people underestimate their own longevity, an annuity can feel like a gamble against dying early, products are perceived as inflexible, and the process itself carries friction, from understanding the market to shopping around and switching provider.

A purely mathematical view asks how many years a client must live for cumulative annuity income to overtake leaving the money invested. In the illustrative example shown, that break-even sat at around 23 years. But Warwick's central point was that break-even misses three things that matter:

Investment tails
A partial annuity cuts off the worst outcomes. It is not about beating the average return, but protecting against a bottom tail a client cannot afford.
Longevity tails
Fear of dying early ignores the roughly equal chance of living well beyond average, which is exactly the risk an annuity insures.
Behavioural value
Covering essential spending brings peace of mind, helps clients calibrate spending, and can free them to be more aspirational with the rest.
A secure income covering the basics can give clients permission to spend, gift and fulfil their aspirations, the idea at the heart of Bill Perkins' "Die With Zero".

A rate landscape that has shifted

UK annuity rates track the 15-year gilt yield closely. Three moments stand out: the 2008 financial crisis, after which yields and rates fell sharply; 2015 pension freedoms, which made drawdown relatively more attractive; and the 2022 bond market sell-off, after which yields and rates rose again. Rates are not quite back to their pre-2008 levels, but they are close, and even on a break-even basis alone the case looks more compelling than it has for well over a decade.

The market has responded. UK annuity purchases by value have more than doubled, rising around 106% from roughly £3.6bn in 2022 to £7.4bn in 2025. Thinking has moved on too, with ideas such as building up slices of secure income over time like a glide path, using a deferred annuity from age 75 as genuine longevity insurance, and blending a partial annuity with a portfolio that can then take slightly more risk on the remainder.

The elephant in the room

For an adviser charging on assets under management, annuitising reduces the fund and, all else equal, the revenue it generates. Warwick was candid about that tension, while arguing that under-using annuities is the bigger risk. A thoughtful approach to secure income strengthens the proposition and builds trust precisely because a recommendation that appears to run counter to the adviser's own incentives tends to deepen it. Blended solutions preserve much of the fund, and a partially annuitised portfolio is drawn on at a lower rate, so it depletes more slowly.

The summary: annuities are not for every client, but they should not be overlooked. They are an important part of the retirement toolkit, and advisers are visibly changing their thinking.


Drawdown management and bucketing

Bucketing splits assets into pots to manage sequence risk. A two-bucket version pairs a long-term growth portfolio with a cash reserve: you spend from cash, replenish from growth, and press pause on selling growth assets during a downturn. A three-bucket version adds a medium-term pot, often bonds or a lower-risk multi-asset fund. The session examined it from two angles.

The behavioural case is real

Bucketing taps into mental accounting, the way people naturally think about money in separate pots. As Daniel Kahneman observed, money is fungible even when we treat it otherwise. That framing can strengthen the adviser relationship, encourage clients to accept volatility in the long-term bucket, and support withdrawal discipline. The value of avoiding panic is hard to overstate: in the 2020 sell-off, moving a 60/40 portfolio to cash left an investor materially worse off than one who stayed the course.

"People often treat money differently depending on where it comes from, labelling it in separate mental accounts, even though the money is fungible."
Daniel Kahneman, Thinking, Fast and Slow

The financial case is weaker

The trouble starts when markets move. Steady growth is simple, but a booming or falling market forces judgement calls: take profits into cash, or not? Stop selling growth, and when do you restart? These are market-timing decisions, often made on gut feel, under pressure, and inconsistently across a client bank. Add a third bucket and the permutations multiply.

The most rigorous evidence cited was Javier Estrada's work (IESE Business School, 2019), spanning 115 years of data across 21 countries. It found that bucketing strategies tended to underperform a disciplined static allocation. Bucketing avoids selling low, but it also misses buying low, whereas a rebalanced static portfolio does the opposite, buying into a market fall. Cash itself carries a cost, with a historically negative real return that drags on performance. And when you sketch out what bucketing actually produces over time, lower-risk assets rising with inflation while growth assets deplete, you arrive at something that looks a lot like a glide path, only with discretionary market calls bolted on.

The verdict
A single-portfolio approach is generally more efficient from a portfolio-construction perspective. Bucketing can still earn its place in engaging and coaching clients. If you use it, the discipline is non-negotiable: predetermine your replenishing rules, make them robust, stick to them, and keep an eye on the headline asset allocation. Bucketing is not a magic answer to decumulation on its own.

The opportunity in complexity

None of the three areas solves decumulation by itself, and even together they do not fully tame it. A great retirement experience comes from getting the portfolio, the financial planning and the client relationship working in concert. That is precisely why the problem is such an opportunity. Much of the value advisers add, from behavioural coaching to spending strategy, is concentrated in the decumulation phase.

Sharpe called it the nastiest, hardest problem in finance. Handled well, it is also one of the clearest chances an adviser has to demonstrate their worth.

From the Q&A

Pinning down a client's real spend. The PLSA Comfortable figure is a useful anchor, but most clients want more. Online tools, checklists and category-by-category spreadsheets all help, and estimates should be course-corrected over time as plans meet reality.

Personal inflation for wealthier clients. Planning to a rate above headline CPI can be sensible, balanced against false precision. Where spending is broken down by category, a more personalised inflation assumption can be built up and tends to resonate with clients.

Implementing dynamic spending. A cash flow plan is a powerful route. Rerun it each year: if the sustainable figure moves from £50,000 to £48,000, that is dynamic spending in action, and course correction made concrete.

A middle ground on bucketing. Present the mix of stocks, bonds and cash to clients as buckets for clarity, while managing it behind the scenes as one portfolio, rebalancing into falls.

Inheriting a client drawing down too fast. Harsh truths, delivered gently, with evidence. A stochastic cash flow shows a range of outcomes rather than a single line, which makes a difficult conversation more manageable, if never entirely easy.


Important: This blog is prepared exclusively for use by financial advisers; retail distribution is at the adviser's sole risk and discretion. It does not constitute advice, an offer or a solicitation to invest.

Compiled from sources believed to be reliable. Any views, opinions or estimates expressed, including any forecasts or forward-looking statements, constitute the author's judgment at the time of writing, are not guaranteed and are subject to change without notice. None of Timeline, its directors, officers or employees accepts liability for any loss arising from the use hereof or reliance hereon or for any act or omission by any such person, or makes any representations as to its accuracy and completeness.

Sources and notes
All figures are illustrative and do not reflect the results of any particular investment. Past performance is not a reliable indicator of future results.
Regulatory backdrop: FCA Retirement Income Advice thematic review (TR24/1), March 2024, fca.org.uk.
Spending standards: PLSA Retirement Living Standards, June 2025. Cost of Living Extremely Well index: Forbes, 2024.
Withdrawal-rule and annuity-tail modelling: Vanguard, based on VCMM simulations as at December 2024. UK annuity rates and 15-year gilt yield: williamburrows.com and Bloomberg. Annuity market value: Association of British Insurers.
Bucketing research: Estrada, J. (2019), IESE Business School. Cost-of-panic illustration: Vanguard calculations using Morningstar data.
Presented by Warwick Bloore (Vanguard Advisory Research Centre). Hosted by Jake Usher (Timeline).

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