Following continued audience interest after Episodes 22 and 29, Jeremy takes on a much more detailed listener theory attempting to explain precisely how the "18.6-year property cycle" is supposed to work, and tests it directly against historical data.
A Detailed Listener Theory
A commenter, Thin Yuan, outlines a specific four-part structure to the cycle: a roughly seven-year boom in "primary markets" (Sydney, Melbourne, and their surrounding cities, doubling in value), followed by a multi-year "mid-cycle downturn" (this time, attributed to COVID-19), then a second roughly five-to-seven-year boom in "secondary markets" (Queensland, Western Australia, and South Australia, also doubling in value), before a final correction period. Jeremy notes he separately came across a similar reference to the theory on a UK-based podcast, where a guest cited the same 18.6-year framework without appearing to have independently verified it, illustrating how easily an unverified claim can be repeated as established fact once it's been picked up by one credible-sounding source.
Setting Up the Test
Using the commenter's own definitions, Jeremy defines "primary markets" as Sydney, Melbourne, and their surrounding regional cities (including the Central Coast, Wollongong, and Geelong), and "secondary markets" as the states of Queensland, Western Australia, and South Australia as a whole. Since the theory requires each boom period to double property values, Jeremy calculates the growth rate needed: roughly 9% per annum to double over 8 years, or at least 10% per annum if the boom lasts the full 7 years the theory describes, in each case comfortably qualifying as double-digit annual growth.
Why a Growth-Rate Chart, Not a Dollar-Value Chart
Jeremy explains a key methodological point: some earlier critics of his prior debunking wanted him to use raw dollar-value charts rather than percentage growth-rate charts. He demonstrates why this is actually the wrong approach: on an exponential dollar-value curve, the visually "steepest" part of the chart is misleading, since it typically reflects the most recent, highest-value period rather than the fastest percentage growth rate. He shows directly that the true fastest growth period for the primary markets was actually very early in the dataset (appearing as a barely visible ripple on the dollar chart), the opposite of where it visually appears to be on a dollar-value chart. A percentage growth-rate chart, with a clear 10% benchmark line, allows the precise start and end of genuine double-digit growth periods to be identified, which a dollar chart cannot reliably show.
Testing the Primary Markets
Examining Sydney, Melbourne, and their surrounding cities' combined growth rate from 1980 to 2024 (44 years), Jeremy identifies the points where growth crosses above and below the 10% benchmark line. The gaps between successive "boom start" events come out to intervals of roughly 3, 7, and other clearly inconsistent lengths, nowhere near a repeating 18.6-year pattern, and in 44 years, growth crossed above the benchmark at least four separate times, which alone rules out an 18.6-year repeating cycle mathematically (four cycles of that length would span 74+ years). On boom duration specifically, Jeremy identifies five distinct double-digit growth periods in the primary markets, ranging from as short as one year to as long as nine years, with only one falling within the theory's predicted 5–7 year range.
Testing the Secondary Markets
Applying the same method to the combined Queensland, Western Australia, and South Australia growth rate, Jeremy again finds the "fastest growth" period visually appears at the recent end of a dollar-value chart, but is actually located much earlier once measured correctly as a percentage. Measuring the gaps between successive boom-start events, only two of several measured intervals come reasonably close to 18–19 years, the remaining cases fall well outside that range. On boom duration, secondary market booms lasted as little as roughly 2 years in one case and around 6.5 years in another, again showing no reliable consistency with the theory's predicted 5–7 year window.
Testing the Gap Between Primary and Secondary Booms
The theory also predicts a distinct multi-year gap between the end of the primary market boom and the start of the secondary market boom. Plotting both markets' growth rates on the same chart, Jeremy shows that on at least three separate occasions (including the very start of the dataset and the late 1980s), both primary and secondary markets were actually in boom conditions simultaneously, directly contradicting the theory's predicted sequential gap.
Even If the Timing Were Right, the Theory Still Wouldn't Help You Choose Where to Buy
Jeremy raises a further, separate problem: even setting aside the timing issues entirely, the theory doesn't specify which individual city within the "primary" or "secondary" grouping to invest in. Using the late-1980s primary market boom as an example, growth varied dramatically across the group, Sydney grew around 75%, Wollongong around 65%, Melbourne around 55%, and Geelong only around 40%, nearly double the growth in the best-performing city compared to the weakest. A similar pattern held for a sample of secondary market cities, with roughly a 60 percentage point gap in growth between the best (Perth) and worst (Adelaide) performing markets over the same boom period. Jeremy's point: even if the broader theory's timing were somehow accurate, it still provides no useful guidance on which specific market within each group to actually invest in, a critical gap for any investor trying to act on it.
Why This Kind of Misinformation Spreads So Easily
Jeremy reflects on how a theory like this can spread from a single book into wider circulation, particularly once it's repeated by someone perceived as credible or authoritative (an author, a podcast guest, an "expert"), without independent verification. Both hosts agree there's no shame in having been misled at some point, but suggest the real issue is continuing to hold onto a belief once solid contrary evidence has been presented. Damien reflects on how difficult it can be, practically, for an average reader without access to historical data or analytical tools to fact-check a confidently written book themselves, and how much harder it becomes to walk back a belief once it's already been shared with others.
Closing Thoughts
Jeremy reiterates that across every test applied (Australia nationally, Sydney, Melbourne, Brisbane, the US, the UK, and now specifically defined primary and secondary Australian markets, using both aggregate and individual-city data), no version of the 18.6-year cycle theory holds up against historical data. He remains open to a genuinely data-backed challenge to this conclusion, but reiterates that comments containing opinion alone won't change his view. Both hosts close by encouraging listeners to like, comment, subscribe, and share the episode with anyone who still believes in the cycle.