Handedness is an attribute of humans and several animal species that pertains to preference or skill in one hand over the other with fine motor skills. This unequal distribution of fine motor skills determines which hand an individual may prefer for virtually all human activities, including but not limited to writing, handling objects, and operating machinery. Handedness is correlated to (but does not always correspond with) eye dominance. The two most common types of handedness are right- and left-handedness, with 90-93% of the world's population being right-handed. To be equally proficient with both hands is to be ambidextrous; though many people can perform certain tasks equally well with both hands, true ambidextrousness (the ability to perform all tasks equally well with both hands) is exceptionally rare.
Some people prefer different activities with different hands; for example, one might throw a ball with the left hand but bat a ball right-handedly. This is sometimes classified as cross-handedness, but more often handedness is determined by the hand with which one uses to write; for example, even if one prefers his or her left hand for a majority of activities, if he or she writes with the right hand he or she would be considered right-handed.
Historically (prior to the early 20th century in the United States and even through the late 20th century in some parts of the world), left-handedness has been considered to be a disability and teachers have often encouraged children to take up activities with the right hand even if it does not feel natural for a child; this can lead to several documented developmental difficulties. Many recorded instances of ambidextrousness have occurred from originally left-handed people who were forced to take up right-handedness because of societal pressures, and it is still considered rude to perform certain tasks with the left hand (such as eating or shaking hands) in many parts of the world. Because so few people in the world are left-handed, a vast majority of objects in the world are designed for use by right-handed people exclusively (for example, the text on a pen is usually printed so that it is readable when the pen is held with the right hand).
The handedness bias is reflected in the languages of the world. In many Western languages, the word for "right" (as in the direction) is often a synonym for "correct" with connotations of authority and justice: recht in German, droit in French, derecho in Spanish, direito in Portuguese, and in several Slavic languages. In many other languages, "right" is associated with skill while "left" is associated with clumsiness: the Latin term for right-handed is dexter as in dexterity, diestro in Spanish and destro in Italian both mean "right-handed" and "skillful," and the Irish deas means "right side" and "nice". On the other hand, the French gauche means "left" and "clumsy", as does the Italian word maldestro and the Dutch word links. English, Dutch, German, Bulgarian, and Czech all have some variation of the idiom "to have two left feet" (in some languages two left hands), meaning to be clumsy. The word "ambidextrous" reflects this as well: it comes from the Latin "dexter," so it could be translated as "having two right hands."
In some languages "left" is even associated with the concept of evil. The English word sinister, the Italian word sinistra, and the Spanish word siniestra all come from the Latin word sinestra, which originally meant "left" but often took on the colloquial meaning "unlucky" and in their respective languages all mean both "evil" and "left" (although the latter meaning is archaic in English and somewhat rare in Spanish). The Chinese adjective meaning "left" (左 or "zuǒ") also means "improper."
Seven out of the fourteen United States presidents since 1928 have been either left-handed or ambidextrous; it is difficult or impossible to say how many were before that time because left-handedness was still looked down upon as a disability so any presidents who were left-handed would have presumably made every attempt to hide it. Since (and including) president Gerald Ford, only two presidents have been purely right-handed: Jimmy Carter and George W. Bush; Gerald Ford himself as well as George H.W. Bush, Bill Clinton, and Barack Obama are all left-handed and Ronald Reagan was ambidextrous (and could have been left-hand dominant, but this is debated). Herbert Hoover was the first president to be described as left-handed, and Harry S. Truman was ambidextrous but preferred his left hand. The tradition of left-handed presidents even extends to the elections themselves: in 1992, all three major candidates (George H.W. Bush, Bill Clinton, and Ross Perot) were left-handed; in 1996, Clinton, Perot, and Bob Dole were all left-handed; and in 2008, both major-party candidates (Obama and John McCain) were left-handed.
This trend is not replicated in other countries. Only two post-war British prime ministers, Winston Churchill and James Callaghan, were left handed (although it has been documented that some British schools were discouraging left-handedness as recently as the 1990s), and Canada has not had a left-handed prime minister since 1980.