The emotional effect of line shape is
probably partly due to cross-sensory
associations such as the bouba/kiki effect:
Which is "Bouba" and which is "Kiki"?
"The bouba/kiki effect is a non-arbitrary mapping between speech sounds
and the visual shape of objects. This effect was first observed
by German-American psychologist Wolfgang Köhler in 1929.
In psychological experiments, first conducted on the island of
Tenerife (in which the primary language is Spanish), Köhler showed forms
similar to those shown at the right and asked participants which shape
was called "takete" and which was called "baluba" ("maluma" in the 1947 version).
Although not explicitly stated, Köhler implies that there was a strong preference
to pair the jagged shape
with "takete" and the rounded shape with "baluba".
In 2001, Vilayanur S. Ramachandran and Edward Hubbard repeated Köhler's
experiment using the words "kiki" and "bouba" and asked American college
undergraduates and Tamil speakers in India "Which of these shapes is
bouba and which is kiki?" In both groups, 95% to 98% selected the curvy shape
as "bouba" and the jagged one as "kiki"."
From 
