In his book The Black Swan: The Impact of the Highly Improbable, risk engineering expert Nassim Taleb describes a black swan as "an event, positive or negative, that is deemed improbable yet causes massive consequences." 

What makes negative black swan events so dangerous is that they can't be predicted en masse and therefore priced in by investors. Because the odds are seriously stacked against investors predicting the next black swan event, they're likely better served by focusing on the major known external risks that could derail a long-term investment thesis for a stock or industry.  

In the world of 3D printing, three major external risks now threaten 3D Systems (NYSE:DDD) and Stratasys (NASDAQ:SSYS) -- the two largest companies in the space.

1. Capital spending dries up
When a 3D printing company cites a weakening capital spending environment as cause for slower than expected revenue growth, it's really saying customers delayed or canceled orders due to external factors. Whether it's a recession, currency headwinds, market saturation, or something totally unforeseen, if capital spending dries up for 3D printing, it would invariably have negative consequences for 3D Systems and Stratasys.

Unfortunately, both 3D Systems and Stratasys experienced capital spending weakness in North America -- 3D printing's largest market -- in the first quarter. 3D Systems attributed the softening to collapsing oil prices and the strong U.S. dollar, which caused customer uncertainty and prompted order delays from healthcare, automotive, and oil and gas customers.

Stratasys didn't provide a clear-cut explanation beyond stating there was a "relevant decline within certain regions and industries, particularly in North America," but it's likely that its headwinds were similar to those faced by 3D Systems, considering both companies target the same industries.

2. Disruptive technologies
By the end of next year, two potentially disruptive 3D printing technologies are expected to come to market: CLIP from Carbon3D and Multi Jet Fusion from Hewlett-Packard (NYSE:HPQ). Ultimately, these technologies threaten to undermine 3D Systems' and Stratasys' 3D printing technology portfolios.  

Carbon3D's CLIP technology promises to be 25 to 100 times faster than existing 3D printing technologies and features part qualities that resemble injection molding in terms of smoothness, thanks to an innovative continuous 3D printing process.

Hewlett-Packard's Multi Jet Fusion is claimed to be up to 10 times faster than leading material extrusion and selective laser sintering technologies on the market today. The goal of Hewlett-Packard's 3D printer is to create a step-change within the 3D printing industry so that it can capture a large share of the potential addressable market.

According to J. Scott Schiller, worldwide director of HP 3D printing, there's an order-of-magnitude difference between current 3D printing adoption rates and potential 3D printing adoption rates. In Schiller's view, the way to bridge this gap is to create a 3D printing technology platform that, over time, becomes faster, produces higher-quality parts, operates at a lower cost, and is highly predictable for engineers. From this perspective, it seems almost inevitable that Hewlett-Packard's Multi Jet Fusion printer is only the starting point of the tech giant's 3D printing reign.

3. Falling prices
In an environment in which competition is slated to intensify in the coming years, the result could be falling 3D printer prices, which in turn could eat into 3D Systems' and Stratasys' profitability. This is particularly true after considering that breakthrough technologies such as CLIP and Multi Jet Fusion will likely make other 3D printing technologies appear inferior in many ways.

Ultimately, the best course of action for 3D Systems and Stratasys to mitigate the threat of pricing pressure is to drive innovation through research and development that meets or exceeds CLIP's and Multi Jet Fusion's capabilities. Obviously, this is easier said than done, and is far from a guarantee.

Putting it all together
The irony of black swan events is that they're near-impossible to predict, but could expose investors to significant risk. Because no one can say with certainty what the next black swan will be or when it will hit, it's probably best for investors to focus on the major external risks that threaten to undermine an investment thesis. For 3D Systems and Stratasys, that would entail focusing on the health of the capital spending environment, threat of disruptive technologies, and 3D printer prices.