What was the longest lasting economic bubble?

Let's assume that you are actually asking about a bubble, not a boom. (because if you were asking about a boom, the question would be trivial, since the answer is on Wikipedia).

Warning - I'm going to argue that this is a wicked problem - the answer is entirely dependent on which assumptions you treat as credible at the start, and that working with different assumptions, different people can, through rigorous analysis, come to different answers. Unless you clarify the assumptions, the answer is no more useful than "Is Nicole Kidman prettier than Gillian Anderson?" As five different people and you'll get seven different answers, and all of them will be equally right.

What is the definition of a bubble?

The term "bubble," in the financial context, generally refers to a situation where the price for an asset exceeds its fundamental value by a large margin. Investopedia

A bubble is an economic cycle characterized by rapid escalation of asset prices followed by a contraction. It is created by a surge in asset prices unwarranted by the fundamentals of the asset and driven by exuberant market behavior. When no more investors are willing to buy at the elevated price, a massive selloff occurs, causing the bubble to deflate. Investopedia

Couple of critical features:

1. There can be a bubble in anything - from tulip bulbs to beanie babies to real estate to stocks. So there is probably a bubble going on right now in something.

2. A bubble is defined as a surge in prices unrelated to the fundamentals of value. But there is no objective standard of value. Recently money moved from equities to gold and bitcoin because of rumors of conflict with North Korea. Was that surge of value in bitcoin justified by the fundamentals of bitcoin? Or was the surge unrelated to the conflict, and the surge was actually caused by rising trust in the stability of bitcoin after it avoided the worst effects of the bitcoin fork? What is the true value of bitcoin and what portion of the surge was reasonable and what portion was unreasonable. For commodities that are studied widely, it is possible to achieve a consensus estimate on when the bubble starts and ends - this is not an objective estimate, but a consensus that from point X to point Y there was a bubble.

3. We can generally point to the end of a bubble because they tend to end in a crash. But it is difficult to point to the beginning. Unreasonable valuation always begins from reasonable valuation. People buy a commodity because they feel it is undervalued; as the value rises it crosses the value estimation of various buyers, until there are fewer buyers than sellers.

isplacement: A displacement occurs when investors get enamored by a new paradigm, such as an innovative new technology or interest rates that are historically low. A classic example of displacement is the decline in the federal funds rate from 6.5% in May, 2000, to 1% in June, 2003. Over this three-year period, the interest rate on 30-year fixed-rate mortgages fell by 2.5 percentage points to a historic lows of 5.21%, sowing the seeds for the housing bubble. investopedia

Combine the above, and I can almost guarantee you that there is a long bubble in some commodity right now, and nobody is paying any attention to it. So the question becomes very difficult to answer - easy to argue about, but difficult to authoritatively answer.

IF I had to make a guess, I'd advocate the housing bubble. Although we normally refer to the housing bubble of 2007-2008, the real housing bubble started probably back in the 1970's -

• High inflation made it a prudent thing to borrow money - you were sure to pay back the loan with money that was cheaper to earn than current money.

• Government backing of real estate loans kept the price low and the availability high. by law (or more precisely by government policy) the value of real estate was disconnected from the fundamental valuation.

• Tax policy artificially inflated the value of real estate. Fairly simple economic models show that the price of housing rose in direct proportion to the value of the tax break on real estate. A 100K house with a 10% tax advantage suddenly cost 110K. (the actual model is a bit more complex and has to take into account regional variations in income, supply, demand, etc.) Again, the government intentionally disconnected the value of housing from the fundamentals of housing. They did this to bolster the middle class dream of owning a house, but the actual effect was to make housing unaffordable for the lower income strata of society.

• Other policies (VA loans, fiscal deregulation, etc.) further inflated the value of housing away from the fundamentals.

NOTE: I said arguably My point is that every statement of the longest bubble is going to be argued by someone - I'm skeptical of any objective authoritative answer. But OP asked for the longest bubble - I'd argue that the 30 year bubble in housing is a good candidate.

Of course, OP didn't specify in which economy; if we looked at the world economy, I'd argue that the valuation of the dollar was divorced from the fundamentals of the dollar from 1945 to 2007 - which is a longer bubble. Someone else could probably argue for some other commodity - wine perhaps? Oil (where cartel politics divorce the fundamentals from the value).

Of course, if you had intended to ask about a stock market bubble, you would have specified that in your question.

Opinions differ. This man argues that we're in a bubble. I'm sure if I spent 5 minutes searching, I could find someone to take exactly the opposite opinion. Over in the corner, you'll see the gold bugs frothing at the mouth and chanting "fiat money is evil" and insisting that we've been in a bubble since Bretton Woods; fortunately most of them are now easily identifiable by their foil hats, and the fact that their chants have diminished to a weary mumble as reality moves further and further from their ideological purity. (Stop for a moment and ponder the fact that the supply of gold in the world is fixed while both population and productivity continue to rise at a greater than linear rate). (let me pause here and take my tranquilizers - the mention of gold bugs has a negative effect on my blood pressure).

When the market corrects, there will be a host of people shouting that this was evidence of a bubble; others will insist just as forcefully that it was a correction.

So here is the real question - In what context is the answer to the question useful? What difference would it make if you accepted my argument that housing was the longest bubble? Or the raving loon in the corner's assertion that fiat money is the bubble? Or bitcoin?