If you were told that these two shapes had names, and those names were Bouba and Kiki, which shape do you think would be Bouba and which one Kiki?
Think about it. Don't continue reading until you've made a decision.
No. Not yet. Think about it. Which shape is named Bouba? Which shape is named Kiki?
Okay, now to the results.
If you picked Kiki as the left-hand shape and Bouba as the right hand shape, then you have made the choice that between 95-98% of people made. That's right, 95-98% of people choose Kiki as the left hand shape and Bouba as the right hand one.
This is explained in more detail at the Wikipedia article Bouba/kiki effect:
In tests conducted with both English and Tamil speakers, 95% to 98% picked the curvy shape as bouba and the jagged one as kiki, suggesting that the human brain is somehow able to extract abstract properties from the shapes and sounds. Recent work by Daphne Maurer and colleagues has shown that even children as young as 2.5 (too young to read) show this effect.So words are not necessarily arbitrary - objects can actually create abstract concepts in the human mind. Fascinating captain.
Ramachandran and Hubbard suggest that the kiki/bouba effect has implications for the evolution of language, because it suggests that the naming of objects is not completely arbitrary. The rounded shape may most commonly be named bouba because the mouth makes a more rounded shape to produce that sound while a more taut, angular mouth shape is needed to make the sound kiki. The sounds of a K are harder and more forceful than those of a B, as well. The presence of these "synesthesia-like mappings" suggest that this effect might be the neurological basis for sound symbolism, in which sounds are non-arbitrarily mapped to objects and events in the world.
Interestingly, individuals afflicted with autism do not show this effect. Where average people agree with the typical result 90% of the time, autistics only agree 60% of the time.