Hands - Research
Thousands of scientific studies have been undertaken on hands. Some aspects are 'proven'. For example, science has shown that certain hand features or line formations are more common in particular syndromes or health conditions.

Medical doctors look at the hands as an aid to diagnosis, e.g. shape, colouration and markings on fingernails being associated with various medical conditions.
Many studies have been done on the hand in different chromosomal disorders, such as studying the dermatoglyphics ("skin carvings") - skin ridges, fingerprints, palmar patterns.
The Single Transverse Palmar Crease ("Simian Line") has been found to be more common in syndromes such as Down's (65%), Patau's, etc than in the general population (2-5%). Over the last few years there have been scientific studies into subjects such as the hand and fertility, diabetes, autism, heart disease, schizophrenia etc. Recently scientists are beginning to study aspects of the personality & psychology in conjunction with hands.

The scientific studies about hands are too numerous to mention here. If you go to PubMed, National Library of Medicine you can search there. They have thousands of abstracts of scientific papers. Search using the scientific terms rather than palmistry nomenclature. For example, if you want to see what research has been done on the skin ridges ie fingerprints and palmar patterns, a search on "dermatoglyphics" reveals almost 5000 such studies (10 years ago it was showing 4000, which kinda shows how the research is increasing in recent years). If you want to know about the Simian line, enter "Single Transverse Palmar Crease" or "Simian crease" for your search. Or you might want to look up the work of a particular scientist, eg John Manning's research into relative digit length, many studies have been done on 2D:4D ratio.
Remember, not all scientific studies are accurate! Some studies, on closer inspection, have their flaws. But if handreaders become a bit more interested in science, and scientists maintain an interest in hands, in the future many more aspects of handreading will be backed up by empirical evidence. The bridge between handreading and science is already being built! For more information about hand research, see this excellent website "Handresearch" - Martijn van Mensvoort's site