The absence of double bonds decreases fluidity, making the membrane very strong and stacked tightly. Unsaturated fatty acids have at least one double bond, creating a "kink" in the chain. The double bond increases fluidity. Membrane fluidity is also affected by cholesterol.

What would decrease the fluidity and permeability of a cell membrane?

In mammals, cholesterol increases membrane packing to reduce membrane fluidity and permeability. The fatty acids tails of phospholipids also affect membrane fluidity. Fatty acids can vary in length, and the number of double bonds in the hydrocarbon chain.

What causes fluidity of the cell membrane?

The mosaic nature of the membrane, its phospholipid chemistry, and the presence of cholesterol contribute to membrane fluidity.