Professional costumer and textiles worker here. This is a fairly thorough and accurate explanation. I would also add that the motion that fabrics experience in the process of machine washing and drying also stretches, pulls, and contracts the fibers, and The tensile strength of the fiber as well as its flexibility determines how it reacts to those processes as well. The other factor in how a garment reacts to washing is what it’s made of and how it’s produced.

TL/DR: RTFM. If your clothes say machine wash, dry flat on the tag, follow those directions if you like your clothes.

In-depth explanation:

Natural fibers like cotton, linen, and silk tend to be more resilient. This is partly because they have already evolved to survive the elements like heat, cold, and water, and partly because they are fibrous in their natural state.

Synthetic fibers, like nylon, rayon, and spandex tend to have more give because of the elasticity of their chemical makeup, however as they are manufactured from plastic, their natural state is one of a solid or liquid, and heat tends to break down the artificial bonds which have made them into a fiber. That’s why your rayon or spandex garments will often recommend an air dry or low heat.

The method of production also has an effect on a garment’s resilience.

Woven fabric, which is made on a loom in the way we tend to think of fabric being made (right? I’m not the only one who thinks about this stuff? I am? Okay, moving on), tends to be much stronger, and have a lot less give. It is many fibers laid flat and woven across each other to produce a number of different patterns and textures, and tends to be very strong and not very flexible. Your dress shirts, suits, formal dresses, and curtains are all (probably) woven. This means it has very little give in the fit, and if the fibers move too much, they tend to break rather than stretch. This is why these garments often say Dry Clean Only, because the act of subjecting the fibers to the motion of the washing machine will eventually destroy the garment.

Knit fabric, which is manufactured in a number of ways, from complicated knitting machines all the way down to grandmas with needles, takes a single, impossibly long fiber and wraps it in and around itself to create a much more flexible texture. Most modern casual clothing, from your sweaters and leggings, all the way down to your t-shirts and sweatpants, is produced in this way, making it stretchy and comfortable. The directions on these garments have a wide range, but most are machine washable, with instructions on temperature and drying method being variable.

This is where the material comes in. The more synthetic fiber in your garment’s content, the lower its tolerance for heat will be.

The exception to this rule is garments made from yarn, like sweaters, hats, and scarves. Yarn is many fibers spun together to make one thick fiber that is used to make the garment. Now matter how natural the wool used to make the yarn, it still won’t tolerate heat and twisting well. Think of a fiber optics cable, where the individual filaments are soft and flexible, but you really don’t want to twist and bend the cable too much or you’ll damage its fidelity.

If you’re still with me, thanks for reading. This is one of my areas of expertise and I’m glad someone on the Internet needed it today.