"All living beings have Kamma as their own."
-- MAJJHIMA NIKĀYA
Kamma  is the law of moral causation. Rebirth is its corollary. Both Kamma and Rebirth are interrelated, fundamental doctrines in Buddhism.
These two doctrines were prevalent in
What is the cause of the inequality that exists amongst mankind?
How do we account for the unevenness in this ill-balanced world?
Why should one be brought up in the lap of luxury, endowed with excellent mental, moral, and physical qualities, and another in absolute poverty, in abject misery? Why should one be born a millionaire and another a pauper? Why should one be a mental prodigy and another an idiot? Why should one be born with saintly characteristics and another with criminal tendencies? Why should some be linguists, artists, mathematicians, and musicians from the very cradle? Why should others be congenitally blind, deaf, and deformed? Why should some be blessed and others cursed from their birth?
Either there is a definite cause for this inequality or there is not. If there is not, the inequality is purely accidental.
No sensible person would think of attributing this inequality to blind chance or pure accident.
In this world nothing happens to any person that he does not for some reason or other deserve. Usually the actual reason or reasons cannot be comprehended by men of ordinary intellect. The definite invisible cause or causes of the visible effect may not necessarily be confined to the present life, but could be traced to a proximate or remote past birth. With the aid of telesthesia and retrocognitive knowledge, may it not be possible for a highly developed seer to perceive events which are ordinarily imperceptible to the physical eye? Buddhists affirm such a possibility.
The majority of mankind attribute this inequality to a single cause such as the will of a Creator. The Buddha explicitly denies the existence of a Creator as an Almighty Being or as a causeless cosmic force. 
Now, how do modern scientists account for the inequality of mankind?
Confining themselves purely to sense-data, they attribute this inequality to chemico-physical causes, heredity, and environment.
Julian Huxley, a distinguished biologist, writes:
"Some genes control colour, others height or weight, others fertility or length of life, others vigour and the reverse, others shape or proportions. Possibly all, certainly the vast majority, of hereditary characteristics are gene-controlled. For mental characters, especially the more complex and subtle ones, the proof is more difficult, but there is every evidence that they are inheritable, and no evidence that their inheritance is due to a different mechanism from that for bodily characters. That which is inherited in our personality and bodily peculiarities depends somehow upon the interaction of this assorted battery of genes with which we are equipped at fertilization. "
One must admit that all such chemico-physical phenomena, revealed by scientists, are partly instrumental, -- but could they be solely responsible for the subtle distinctions that exist amongst individuals? Yet, why should identical twins who are physically alike, inheriting like genes, enjoying the same privileges of upbringing, be temperamentally, intellectually and morally totally different?
Heredity alone cannot account for these vast differences. Strictly speaking, it accounts more plausibly for some of the similarities than for most of the differences.
The infinitesimally minute chemico-physical germ, which is supposed to be about 30 millionth part of an inch across, inherited from parents, explains only a portion of man, his physical foundation. With regard to the more complex and subtle mental, intellectual, and moral differences we need more enlightenment. The theory of heredity cannot satisfactorily account for the birth of a criminal in a long line of honourable ancestors, for the birth of a Saint in a family of evil repute, for the arising of infant prodigies, men of genius and great spiritual teachers.
Dealing with this question of heredity, Dr. Th. Pascal writes in his interesting book on 'Reincarnation' :
"To return to the role played by the germ in the question of heredity we repeat that the physical germ, of itself alone, explains only a portion of man; it throws light on the physical side of heredity, but leaves in as great darkness as ever the problem of moral and intellectual faculty. If it represented the whole man, one would expect to find in any individual the qualities manifested in his progenitors and parents -- never any other; these qualities could not exceed the amount possessed by the parents, whereas we find criminals from birth in the most respectable families, and saints born to parents who are the very scum of society. You may come across identical twins, i.e., beings born from the same germ, under the same conditions of time and environment, one of whom is an angel and the other a demon, though their physical forms closely resemble each other. Child prodigies are sufficiently numerous to trouble frequently the thinker with the problem of heredity. In the lineage of these prodigies has there been found a single ancestor capable of explaining these faculties, as astonishing as they are premature? If, to the absence of a cause in their progenitors is added the fact that genius is not hereditary, that Mozarts, Beethovens and Dantes have left no children stamped from birth as prodigies or genius, we shall be forced to the conclusion that, within the limits it has taken up, materialism is unable to explain heredity. Nor is heredity always realized; many a physical characteristic is not reproduced, in families tainted with dangerous physiological defects, many children escape the evil, and the diseased tendencies of the tissues remain latent in them, although they often affect their descendants. On the other hand extremely divergent mental types are often met with in the same family,  and many a virtuous parent is torn with grief on seeing the vicious tendencies of the child. So we find that heredity and environment either fail to fulfill their promise or else give what was not theirs to give."
According to Buddhism this inequality is due not only to heredity, environment, "nature and nurture",  but also to the operation of the law of Kamma or, in other words, to the result of our own inherited past actions and our present doings. We ourselves are responsible for our own happiness and misery. We create our own heaven. We create our own hell. We are the architects of our own fate.
The Cause of Inequality
Perplexed by the seemingly inexplicable, apparent disparity that exists amongst humanity, a young truth-seeker named Subha approached the Buddha and questioned him regarding it.
"What is the reason, what is the cause, O Lord, that we find amongst mankind the short-lived (appāyukā) and the long-lived (dīghāyuka), the diseased (bavhābādhā) and the healthy (appābādhā), the ugly (dubbannā) and the beautiful (vannavantā), the powerless (appesakkā) and the powerful (mahesakkā), the poor (appabhogā) and the rich (mahābhogā), the low-born (nīcakulinā) and the high-born (uccakulinā), the ignorant (duppa257;) and the wise (paantā)?
The Buddha's reply was:
"All living beings have actions (Kamma) as their own, their inheritance, their congenital cause, their kinsman, their refuge. It is Kamma that differentiates beings into low and high states. "
He then explained the causes of such differences in accordance with the law of cause and effect.
If a person destroys life, is a hunter, besmears his hand with blood, is engaged in killing and wounding, and is not merciful towards living beings, he, as a result of his killing, when born amongst mankind, will be short-lived.
If a person avoids killing, leaves aside cudgel and weapon, and is merciful and compassionate towards all living beings, he, as a result of his non-killing when born amongst mankind, will be long-lived.
If a person is in the habit of harming others with fist or clod, with cudgel or sword, he, as a result of his harmfulness, when born amongst mankind, will suffer from various diseases.
If a person is not in the habit of harming others, he, as a result of his harmlessness, when born amongst mankind, will enjoy good health.
If a person is wrathful and turbulent, is irritated by a trivial word, gives vent to anger, ill-will and resentment, he, as a result of his irritability, when born amongst mankind, will become ugly.
If a person is not wrathful and turbulent, is not irritated even by a torrent of abuse, does not give vent to anger, ill-will and resentment, he, as a result of his amiability, when born amongst mankind, will become beautiful.
If a person is jealous, envies the gains of others, marks of respect and honour shown to others, stores jealousy in his heart, he, as a result of his jealousy, when born amongst mankind, will be powerless.
If a person is not jealous, does not envy the gains of others, marks of respect and honour shown to others, stores not jealousy in his heart, he, as a result of his absence of jealousy, when born amongst mankind, will be powerful.
If a person does not give anything for charity, he, as a result of his greediness, when born amongst mankind, will be poor.
If a person is bent on charitable giving, he, as a result of his generosity, when born amongst mankind, will be rich.
If a person is stubborn, haughty, honours not those who are worthy of honour, he, as a result of his arrogance and irreverence, when born amongst mankind, will be of low-birth.
If a person is not stubborn, not haughty, honours those who are worthy of honour, he, as a result of his humility and deference, when born amongst mankind, will be of high-birth.
If a person does not approach the learned and the virtuous and inquire what is good and what is evil, what is right and what is wrong, what should be practised and what should not be practised, what should be done and what should not be done, what conduces to one's welfare and what to one's ruin, he, as a result of his non-inquiring spirit, when born amongst mankind, will be ignorant.
If a person does approach the learned and the virtuous and makes inquiries in the foregoing manner, he, as a result of his inquiring spirit, when born amongst mankind, will be intelligent. 
Certainly we are born with hereditary characteristics. At the same time we possess certain innate abilities that science cannot adequately account for. To our parents we are indebted for the gross sperm and ovum that form the nucleus of this so-called being. There they remain dormant until this potential germinal compound is vitalized by the Kammic energy needed for the production of the foetus. Kamma is therefore the indispensable conceptive cause of this being.
The accumulated Kammic tendencies inherited, in the course of previous lives, at times play a far greater role than the hereditary parental cells and genes in the formation of both physical and mental characteristics.
The Buddha, for instance, inherited, like every other person, the reproductive cells and genes from his parents. But physically, morally, and intellectually there was none comparable to Him in His long line of honourable ancestors. In the Buddha's own words, He belonged not to the Royal lineage, but to that of the Ariyan Buddhas. He was certainly a superman, an extraordinary creation of His own Kamma.
According to the Lakkhana Sutta  the Buddha inherited exceptional physical features such as the thirty-two major marks, as the result of his past meritorious deeds. The ethical reason for acquiring each physical feature is clearly explained in the discourse.
It is obvious from this unique case that Kammic tendencies could not only influence our physical organism, but also nullify the potentiality of the parental cells and genes -- hence the significance of the Buddha's enigmatic statement: "We are the heirs of our own actions."
Dealing with this problem of variation the Atthasālini states:
"Depending on this difference in Kamma appears the difference in the birth of beings, high and low, base and exalted, happy and miserable. Depending on the difference in Kamma appears the difference in the individual features of beings as beautiful and ugly, high-born and low-born, well-built and deformed. Depending on the difference in Kamma appears the difference in worldly conditions of beings as gain and loss, fame and disgrace, blame and praise, happiness and misery".
"By Kamma the world moves, by Kamma men
Live; and by Kamma are all beings bound
As by its pin the rolling chariot wheel.
By Kamma one attains glory and praise.
By Kamma bondage, ruin, tyranny,
Knowing that Kamma bears fruit manifold,
Why say ye, 'In the world no Kamma is'. "
Thus, from a Buddhist standpoint, our present mental, moral, intellectual, and temperamental differences are preponderantly due to our own actions and tendencies, both past and present.
Everything is not due to Kamma
Although Buddhism attributes this variation to the law of Kamma, as the chief cause amongst a variety, it does not however assert that everything is due to Kamma. The law of Kamma, important as it is, is only one of the twenty-four causal conditions (paccaya), described in Buddhist Philosophy. 
Refuting the erroneous view that "Whatsoever weal or woe or neutral feeling is experienced, is all due to some previous action (pubbekatahetu)," the Buddha states:
"So, then, owing to previous action, men will become murderers, thieves, unchaste, liars, slanderers, babblers, covetous, malicious, and perverse in view. Thus for those who fall back on the former deeds as the essential reason, there is neither the desire to do, nor effort to do, nor necessity to do this deed or abstain from that deed. 
This important text contradicts the belief that all physical circumstances and mental attitudes spring solely from past Kamma. If the present life is totally conditioned or wholly controlled by our past actions, then Kamma is certainly tantamount to fatalism or pre-determination or pre-destination. One will not be free to mould one's present and future. If this were true, freewill would be an absurdity. Life would be purely mechanical, not much different from a machine. Whether we are created by an Almighty God who controls our destinies and fore-ordains our future, or are produced by an irresistible past Kamma that completely determines our fate and controls our life's course, independent of any free action on our part, is essentially the same. The only difference then lies in the two words God and Kamma. One could easily be substituted for the other, because the ultimate operation of both forces would be identical.
Such a fatalistic doctrine is not the Buddhist law of Kamma.
The Five Niyāmas
According to Buddhism there are five orders or processes (Niyāmas)  which operate in the physical and mental realms.
1. Utu Niyāma, physical inorganic order; e.g., seasonal phenomena of winds and rains, the unerring order of seasons, characteristic seasonal changes and events, causes of winds and rains, nature of heat, etc. belong to this group.
2. Bīja Niyāma, order of germs and seeds (physical organic order); e.g., rice produced from rice seed, sugary taste from sugar-cane or honey, and peculiar characteristics of certain fruits. The scientific theory of cells and genes and the physical similarity of twins may be ascribed to this order.
3. Kamma Niyāma, order of act and result; e.g., desirable and undesirable acts produce corresponding good and bad results.
As surely as water seeks its own level, so does Kamma, given opportunity, produce its inevitable result, -- not in the form of a reward or punishment but as an innate sequence. This sequence of deed and effect is as natural and necessary as the way of the sun and the moon, and is the retributive principle of Kamma.
Inherent in Kamma is also the continuative principle.
Manifold experiences, personal characteristics, accumulated knowledge, and so forth are all indelibly recorded in the palimpsest-like mind. All these experiences and characters transmigrate from life to life. Through lapse of time they may be forgotten as in the case of our experiences of our childhood. infant prodigies and wonderful children, who speak in different languages without receiving any instruction, are note-worthy examples of the continuative principle of Kamma.
4. Dhamma Niyāma, order of the norm; e.g., the natural phenomena occurring at the birth of a Bodhisatta in his last birth. Gravitation and other similar laws of nature, the reason for being good, etc. may be included in this group.
5. Citta Niyāma, order of mind or psychic law; e.g., processes of consciousness, constituents of consciousness, power of mind, including telepathy, telesthesia, retro-cognition, premonition, clair-voyance, clair-audience, thought-reading, and such other psychic phenomena, which are inexplicable to modern science.
Every mental or physical phenomenon could be explained by these all-embracing five orders or processes which are laws in themselves. Kamma as such is only one of these five orders. Like all other natural laws, they demand no lawgiver.
Of these five, the physical inorganic order, the physical organic order and the order of the norm are more or less of the mechanical type though they can be controlled to some extent by human ingenuity and the power of mind. For example, fire normally burns, and extreme cold freezes, but man has walked unscathed over fire and meditated naked on Himalayan snows; horticulturists have worked marvels with flowers and fruits; and Yogis have performed levitation. Psychic law is equally mechanical, but Buddhist training aims at control of mind, which is possible by right understanding and skilful volition. Kamma law operates quite automatically and, when the Kamma is powerful, man cannot interfere with its inexorable result though he may desire to do so; but here also right understanding and skilful volition can accomplish much and mould the future. Good Kamma, persisted in, can thwart the reaping of bad.
Kamma is certainly an intricate law whose working is fully comprehended only by a Buddha. The Buddhist aims at the final destruction of all Kamma.
Kamma-Vipaka (fruit of action) is one of the four unthinkables (acinteyya), states the Buddha in the Anguttara Nikāya.