That means that in order for the stock to keep rising, either Apple will have to show higher earnings, or investors will need to become willing to pay more for each dollar of earnings than they were previously. And Nathan doesn't think either scenario is too likely.
First of all, as the prior chart makes clear the stock has already enjoyed what Nathan terms a "massive sentiment shift," so it may not be high time for a fresh surge of bullish sentiment.
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And when it comes to Apple's profits, Nathan points out that since 2012, the company's gross margin has fallen from 44 percent to 39 percent—a victim, he says, of increased pricing pressure from competitors, and commoditization of Apple's products.
Michael Khouw, primary strategist at Dash Financial, is also skeptical about Apple's prospects at this point.
"One of the biggest justifications for buying Apple here is that we're going to see some form of multiple expansion. What justifies that? Revenue growth or income growth," Khouw said. "We do not expect that this year we're going to see, quarter-on-quarter versus 2012, higher net income than we did then. Although we probably will see marginally higher revenues."
For that reason, "I would keep my eye on the revenue number. If that thing starts to drift off, then I don't see why the multiple would expand at all," Khouw said.