question: Why are we (for the most part) dominant in one hand and not the other?
comment: I’m a pediatric occupational therapist and I work with kids on hand dominance and improving fine motor skills. I don’t have an evidence-based answer to provide, but rather clinical experience to go off of. My response would go something as follows: Essentially, learning fine motor and in-hand manipulation skills (use of the small muscles of the hand) is very difficult. This is a skill that first emerges days after birth with gross movements like opening and closing the hand (eventually for reaching and grasping objects), and continues to develop throughout childhood with the most obvious skill being handwriting. Some children are pre-disposed to have easier acquisition of coordination with both hands, but for the vast majority of us, we have to practice for days, weeks, and months on end for specific hand skills to develop. As the general trajectory for fine motor skill development is to take at least 4-6 years for activities of daily living (brushing teeth, combing hair, wiping) and handwriting skills to develop, this trajectory would take much longer if we were to use both hands for fine motor tasks. That’s not to say it couldn’t develop. Theoretically, you could push a child to use both hands throughout toddlerhood and childhood. However, it is current best practice to encourage hand dominance to facilitate appropriate time frames for developmental milestones. Take that with a grain of salt because I usually work with kids who are developmentally delayed. TL/DR: It takes too long for us to learn to use both hands so we specialize by using one hand.
passages: Cross-dominance
  
  Cross-dominance, also known as mixed-handedness, hand confusion, or mixed dominance, is a motor skill manifestation in which a person favors one hand for some tasks and the other hand for others. For example, a cross-dominant person might write with the right hand but throw mainly with the left one.
   Neuroanatomy of handedness
  
  An estimated 90% of the world's human population consider themselves to be right handed. The human brain's control of motor function is a mirror image in terms of connectivity; the left hemisphere controls the right hand and vice versa. This theoretically means that the hemisphere contralateral to the dominant hand tends to be more dominant than the ipsilateral hemisphere, however this is not always the case and there are numerous other factors which contribute in complex ways to physical hand preference.
  
  Section::::Language and speech areas.
   BULLET::::- "Ambidexterity", equal ability to use both hands, is exceptionally rare, although it can be learned. A completely ambidextrous person is able to do any task equally well with either hand. Those who learn it still tend to favor their originally dominant hand.
  
  Section::::Causes.
   In social psychology, the dominant response is "the reaction elicited most quickly and easily by a given stimulus". Increased arousal "enhances an individual's tendency to perform the dominant response". This means that "whatever you are normally inclined to do, you will be even more strongly inclined to do when in the presence of others". In an easy task, the dominant response is usually correct or successful, while in a more difficult (complex or unfamiliar) task, the dominant response is often unsuccessful or incorrect. For example, in a simple maze where the only path is a straight line, the dominant (and correct) response would be to run straight ahead. However, in a more complex maze in a cross shape where the start point and goal point are adjacent to each other, running straight ahead (the dominant response) is the incorrect response.
   One common theory as to how handedness affects the hemispheres is the brain hemisphere division of labor. Since speaking and handiwork require fine motor skills, its presumption is that it would be more efficient to have one brain hemisphere do both, rather than having it divided up. Since in most people, the left side of the brain controls speaking, right-handedness predominates. This theory also predicts that left-handed people have a reversed brain division of labor.
   Handedness
  
  In human biology, handedness is a better, faster, or more precise performance or individual preference for use of a hand, known as the dominant hand; the incapable, less capable or less preferred hand is called the non-dominant hand. Men are slightly more likely to express a strongly dominant left hand than women. Right-handedness is the most common form, and it is estimated that around 90% of the world’s population is right-handed.
  
  Section::::Types.
   Laterality of motor and sensory control has been the subject of a recent intense study and review. It turns out that the hemisphere of speech is the hemisphere of action in general and that the command hemisphere is located either in the right or the left hemisphere (never in both). Around eighty percent of people are left hemispheric for speech and the remainder are right hemispheric: ninety percent of right-handers are left hemispheric for speech, but only fifty percent of left-handers are right hemispheric for speech (the remainder are left hemispheric). The reaction time of the neurally dominant side of the body (the side opposite to the major hemisphere or the command center, as just defined) is shorter than that of the opposite side by an interval equal to the interhemispheric transfer time. Thus, one in five persons has a handedness that is the opposite for which they are wired (per laterality of command center or brainedness, as determined by reaction time study mentioned above).
   "Sharing" requires that the execution of a particular motor task depends on the combined actions of all the components that make up the synergy. Often, there are more components involved than are strictly needed for the particular task (see "Redundancy" below), but the control of that motor task is distributed across all components nonetheless. A simple demonstration comes from a two-finger force production task, where participants are required to generate a fixed amount of force by pushing down on two force plates with two different fingers. In this task, participants generated a particular force output by combining the contributions of independent fingers. While the force produced by any single finger can vary, this variation is constrained by the action of the other such that the desired force is always generated.
   In addition, gestures also relate to dominance perceptions. Carney, Hall, and LeBeau (2005) found that high power individuals were more likely to use gestures, initiate more hand shaking and engage in a higher frequency of invasive touch. Dunbar and Burgoon (2005) found that observers rated only males as more dominant with increased use of illustrator gestures. The researchers also found that males perceived their partner to be less dominant when she used more adapter gestures.
   Co-variation also provides "flexibility and stability" to motor tasks. Considering again the force production task, if one finger did not produce enough force, it could be compensated for by the other. The components of a motor synergy are expected to change their action to compensate for the errors and variability in other components that could affect the outcome of the motor task. This provides flexibility because it allows for multiple motor solutions to particular tasks, and it provides motor stability by preventing errors in individual motor components from affecting the task itself.
   Section::::Correlation with other factors.:Left-handedness and sports.
  
  Interactive sports such as table tennis, badminton and cricket have an overrepresentation of left-handedness, while non-interactive sports such as swimming show no overrepresentation. Smaller physical distance between participants increases the overrepresentation. In fencing, about half the participants are left-handed.
  
  Other, sports-specific factors may increase or decrease the advantage left-handers usually hold in one-on-one situations:
   It is during this stage in the development of fine motor skills that a toddler will show hand dominance.
  
  Section::::Developmental stages.:Preschool.
   The continuance of a stereotypic behaviour can be explained by its impact on the basal ganglia and the establishment of a habit in its expression. Research in humans suggests that damage to the dorsal basal ganglia can make it very difficult for a person to switch between two motor behaviours, thus they will continuously perform the same behaviour. Similar difficulties in switching between motor behaviours have been seen in bank voles which suggests that basal ganglia dysfunction might be a factor in the development of stereotypies in rodents and other animals.
   Most areas of speech processing develop in the second year of life in the dominant half (hemisphere) of the brain, which often (though not necessarily) corresponds to the opposite of the dominant hand. 98% of right-handed people are left-hemisphere dominant, and the majority of left-handed people as well.
  
  Section::::Older models.
   Section::::Complications.:Conflicting evidence.
  
  A number of asymmetrical findings have been disputed, with various studies stating null results in opposition to previously reported differences. This is an issue in handedness neuroscience, as imaging methods are highly susceptible to type 1 errors due to the number of comparisons which they make.
  
  Section::::Complications.:Complexity of causality.
   The corticospinal tract is a bundle of white matter which connects the cerebral cortex with motor neurons in the spinal cord. Notably, humans show a natural asymmetry between left and right tracts, such that the left tract (and therefore connections to the right hand) is significantly larger. However this asymmetry has been found to be reduced in left-handers, suggesting a less biased connection to both hands.
  
  Section::::Forced handedness.
   It can also refer to mixed laterality, which refers to a person favoring eyes, ears, feet, or hands on one side of the body. A person who is cross-dominant can also be stronger on the opposite side of the body that they favor; for example, a right-handed person can be stronger on the left side. Cross-dominance can often be a problem when shooting or in activities that require aim, although athletes can still achieve success in sports that require accuracy, like passing in American football and shooting in basketball.
  
  Section::::In baseball.
   Section::::Synopsis.
  
  In June 2005, Nicholas Humphrey receives a call from Dr. John Skoyles who has seen an unpublished paper by Turkish Professor Uner Tan that focuses upon hand dominance in the family. 
   The surface area of the central sulcus has been found to be larger in the dominant hemisphere, as well as the 'hand knob', an area in the primary motor cortex which is responsible for hand movements, is located more dorsally in the left hemisphere of people who are right- compared to left-handed
  
  Section::::Motor areas.:Right shift theory.
   Also notice that the more widely adopted left-foot braking technique in modern race cars requires a sensitive left foot to operate and a left-hand-driver with a more dominated left foot may brake better than those who brakes with right foot or whose left foot isn't dominated, this drastically shorten the time from maximum acceleration to maximum deceleration and left footer possesses a much greater advantages in racing.
  
  Section::::Sports.:Racket sports.
   The relationship between handedness and its neuronal correlates is complex. Language areas themselves are not concretely correlated, and motor area show exceedingly subtle differences. Because of this, the literature shows many differing opinions. Clearly, advances in research are still necessary to unveil true causal relationships between structural differences and their manifestation in the form of handedness.
   Section::::In snooker.
  
  In snooker, swapping the cue from one hand to the other in order to gain easy access to an oblique shot was long thought to be disrespectful, though more recently it has come to be accepted, especially since Ronnie O'Sullivan has dominated the world game and often escaped from snookers by switching to a left-handed action. There are also several players, most notably Barry Hawkins, who play with the opposite hand of their dominant one.
  
  Section::::Famous people.
   A 2014 study attempting to analyze the biological markers of asexuality asserts that non-sexual men and women were 2.4 and 2.5 times, respectively, more likely to be left-handed than their heterosexual counterparts.
  
  Section::::In culture.
  
  Many tools and procedures are designed to facilitate use by right-handed people, often without realizing the difficulties incurred by the left-handed. John W. Santrock has written, "For centuries, left-handers have suffered unfair discrimination in a world designed for right-handers."
   Section::::In other animals.
   Section::::Leader traits.:Distal (trait-like) vs. proximal (state-like).
  