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I would like to know if I need to keep a fraction of my assets in cash or if I can simply invest 100% into stocks. Here is my thinking:

As a young person I'd like to keep most money in stocks and maintain a long holding period. Of course, I might need money unexpectedly. I can now either keep an "emergency fund" in cash, or I could just sell stocks when the need arises.

Selling stocks might come at a bad time when the market is down. But stock prices are supposed to be random (or at least untradeable) according to the Efficient Market Hypothesis. Would this not mean that it does not matter when you sell? I could just sell at any time without any (statistical) repercussions.

I'm sure I have mixed up a few things here so I'm happy to learn.

boot4life
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Very few people would claim that the efficient markets hypothesis holds at all times. Markets are also driven by animal spirits, which means that for some periods markets will be unreasonably fearful or unreasonably enthusiastic. Consider the global stock market at the beginning of the SARS-COV-2 pandemic. Early on in the pandemic stock prices took major hits, almost across the board. A few months later many stock prices were hitting all time highs. If you had happened to need to tap your emergency fund at the bottom of the market, you would have been very sad a few weeks later.

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The general advice regarding non-investment funds:

  • What you need to pay for the expected
  • What your need to absorb the life happens events
  • What you need in an survive a job loss of 3 to 6 months.

Of course, I might need money unexpectedly. I can now either keep an "emergency fund" in cash, or I could just sell stocks when the need arises.

as you point out

Selling stocks might come at a bad time when the market is down. But stock prices are supposed to be random (or at least untradeable) according to the Efficient Market Hypothesis. Would this not mean that it does not matter when you sell? I could just sell at any time without any (statistical) repercussions.

Now you are mixing multiple things. Life happens to you. That means that your need for money may be unrelated to what happens to the market. Your car needing to be replaced this week, because the old one needs a multi-thousand dollar repair just to get running.

If you have to sell shares in your investments now, you could be locking in gains or losses. You could be triggering taxable events.

In the spring of 2020 as the markets cratered. Many people lost their jobs. But many did not. Selling shares then to survive the multiple weeks between their last paycheck and their first unemployment check would have locked in losses. One jurisdiction near where I live saw more applications in one week than they had in the previous 52 weeks combined. The delay was months. Under your plan you would have been forced to sell. Remember nobody knew how long the delay would be or when the markets would come back.

The markets were efficient. They were reacting because people were panicking. There were businesses closing. There were shortages.

The idea of the life happens and emergency funds is to insulate that money from the market. They become a bedrock of safety. You know they will always be there if you need them.

mhoran_psprep
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