There is a relationship between the ratio of marrow to bone skin and the longitudinal Iranian Pistachio smile of pistachios. In either case, the ratio of bone marrow to bone skin for a perfectly smiling fruit is larger than a fruit with a pointed or unsmiling smile. The interesting thing is that there is no significant difference between fruits with a pointed smile and fruits with a smile.

A finding based on anatomical signs shows that smiling-tip pistachios and long-smiling pistachios have different mechanisms. In addition, in smiling fruits, the ratio of the diameter of the brain to the bony skin on the small axis, ie on the location of the longitudinal bulge, is consistently higher. This means that in perfectly smiling fruits, the size of the kernel is larger than the size of the bony skin, in the direction that the force is applied to the inside of the bony skin and tends to drive and separate the two parts of the bony skin. These correlations indicate the presence of mechanical force created by brain growth on the bony skin wall as a mechanism for smiling. It should be noted that there is no experimental evidence to confirm these results.