Extracts from:

Sanity and Insanity

By Charles Mercier, MB. Lecturer in Insanity at the Westminster Medical School, and at the Medical School for Women". (London 1890)

Charles Arthur Mercier 1852-1919


The Causes of Insanity


5.1 Insanity is, in mathematical terms, a function of two variables. That is to say, there are two factors, and only two, in its causation ; and these factors are complementary. Both enter into the causation of every case of insanity, and the stronger the influence of one factor, the less of the other factor is needed to produce the result. These two factors are, in brief, heredity and stress.

It has been explained that, in order to work efficiently, the nervous system should have in a high degree a certain form of instability - an instability which allows of ready and free rearrangement of the atoms of its molecules, with easy and copious liberation of the energy accumulated in them. So long as this rearrangement proceeds without actual decomposition of the molecule, the process remains within the limits of the normal; but it is easy to see that this form of instability has a certain relationship to another form - a form in which the disturbance of the molecules does not stop short at rearrangement, but goes on to partial or even to total decomposition.

Since a certain degree of instability of the first form is an essential to nervous action, and necessarily exists in every possessor of a nervous system, and since this form of instability is related, more or less closely, to the second form, it is obvious that some tendency to the second form of instability exists in every individual. The amount or strength of the tendency varies with each individual, but in every one it exists to some degree.


5.3 Now insanity is a disorder of the highest nervous centres; that is to say, a derangement of the structure of these centres, and this derangement of structure will be produced by slight disturbances where the structure is loosely compacted and the instability great; while in cases in which the structure is well and soundly constituted and of firm stability, it will require a violent disturbance to upset its equilibrium.

... insanity... needs for its production a certain instability of nerve- tissue, and the incidence of a certain disturbance. When the instability of tissue is great, a small disturbance will suffice. When the instability is small, a violent disturbance is necessary. But for every individual... there is a breaking-point. ... if you subject a man to stress, a certain stress will be found, varying with the stability of his nervous system, at which the man will become insane.

Hence, to determine the causes of insanity, we have to find, first the factors which tend to initial stability or instability of the highest nervous arrangements; and, second, the nature and severity of the stresses to which these arrangements are subject.

5.4 The fact that the majority of people do not become, or do not remain, insane, indicates that they possess a nervous organisation of sufficient stability to withstand such stresses as they are subject to. And the minority who become or remain insane, are endowed with a nervous organisation which is either more easily upset, or is subject to stress of greater severity.

As a matter of fact the factor which is chiefly and most often at fault is the nervous organisation. Although, as has been said, there is an intensity of stress which will permanently upset even the most stably constituted nervous system, yet stresses of this extreme severity are so rare in human experience, that in practice a person of normal and average nervous constitution will not be driven mad by any of the ordinary vicissitudes of life. And since, in the vast majority of cases of insanity, we find that the occasion of the disorder was some stress of but medium intensity, we may be quite sure that in all such cases the important factor in the production of the insanity is not the magnitude of the disturbance, but the fragility of the arrangements on which the disturbance breaks.

5.5 The stability or instability of a person's highest nervous arrangements depends primarily and chiefly upon inheritance. Every man is the outcome and the product of his ancestry ; and this is true not only of the broad and fundamental characters by which he is animal, by which he is human, by which he is national, by which he betrays the country and the family from which he proceeds ; but extends to the trivial and minutely trivial characters by which he is distinguished from other individuals of his own race, country, and family.

Doubtless every man is to some extent moulded into conformity with cirumstances by the influence of circumstances upon him; but the small amount of new character that circumstances can produce in any individual, in comparison with the characters transmitted to him by his ancestry, may be gathered from the length of time that circumstances can act upon him, in comparison with the aggregate length of time during which the long line of his ancestry have been subject to modification by circumstances.

... Doubtless... if we subject a child to certain conditions of life, we may in the same way modify the arrangement and alter the stability of its most elaborate and most highly-organised parts - of its highest nervous centres; but... for the great majority of people, the question of the stability or instability of their highest nervous arrangements resolves itself into a question of the kind and degree of organisation that they have inherited from their ancestry.

5.6 ... The laws of heredity are two: the law of inheritance, and the law of sanguinity. Both of them are important in connection with the causation of insanity.

5.7 The Law of Inheritance is simple, and is easily stated and understood. It is that the offspring tend to inherit every attribute of the parents, or that every attribute of the parents tends to appear in the offspring, and will appear unless some counteracting influence prevents. Doubtless there are many cases in which attributes of the parents fail to appear in the offspring, but these are not exceptions to the law. The laws of nature know of no exception, and when apparent exceptions occur, it is either because the laws are acting in ways not understood, or because of the interference and counteraction of other laws. The rising of a balloon is not an exception to the law of gravity, it is an illustration of the law acting in an unusual way.

5.8 That the law is true of the general and broad outlines of structure and function is... universally accepted... But that the same law is true of the smaller attributes, down to the most trivial details of structure and function...

... there are family similarities in handwriting as strong and as frequent as family similarities of features; and this is the more important from the present point of view, since a peculiarity of handwriting depends upon an arrangement of nerve-tissue that must be extremely delicate, extremely elaborate, subtle, slight, diffused, and yet precise. In comparison with such a quality of nervous arrangements, the peculiarity of tissue organisation which underlies insanity is gross indeed; and hence if the one is transmissible by inheritance we may be quite sure that the other may be.

That the children of insane parents are apt to inherit a tendency to insanity is what might be expected, and is a well-established fact ; the existence of insanity in other members of the family being ascertained to exist is more than twenty per cent, of the patients admitted into asylums in this country. But this direct inheritance of insanity is by no means the only way in which the first law of heredity influences the tendency to insanity. What is inherited from an insane person is not insanity itself, it is an undue instability of nervous organisation ; and hence, whenever there is undue instability of nervous organisation in the progenitors, there will be liability to insanity in the offspring.1

5.9 Thus, among the most conspicuous instances and evidences of undue instability of organisation of the higher nervous centres, is epilepsy.

Epilepsy is a sudden and excessive discharge of nerve elements, beginning usually in the highest regions; and the undue and abnormal instability of nerve elements, which epilepsy displays, may be transmitted by inheritance to the offspring. But in the offspring this instability may not exhibit itself as epilepsy. It may be that, instead of a liability to sudden and excessive discharges, there is a liability to excessive and disorderly discharges of much more gradual character, and the same essential defect which in the parent caused epilepsy may in the offspring underlie insanity.

The links between epilepsy and insanity form, in fact, a continuous chain, as will be shown hereafter, and the peculiarities of brain-tissue which underlie them are allied.

In the same way the hysterical parent may have children who become insane, the nervous organisation which allows of the one disturbance being virtually the same as that which allows of the other.

So with people who are highly eccentric, extremely passionate, or who give other evidence of defect in the organisation of the highest nervous arrangements. Such defects are so nearly allied to that which underlies insanity, that it is as natural for the parents who show one of these defects to have children who exhibit another, as it is for a piebald rabbit to have offspring whose piebald markings are of different shape and extent from those of the parent.

5.10 While, however, the existence of instability in the highest nervous arrangements of the parents undoubtedly facilitates, or, more accurately, increases the chances of, the occurrence of insanity in the offspring, it by no means necessarily follows that the children of such parents will become insane ; nor, on the other hand, are the children of those, whose nervous arrangements are of normal stability, by any means exempt from developing such a character of nervous tissue as may involve their breakdown into insanity under the strain of ordinary circumstances.

The reasons of these exceptions to the operation of direct inheritance from parent to child we have now to discover, and they will be found in those influences which have already been alluded to as interfering with or modifying the first law of heredity.

Briefly stated, these influences are as follow:-

5.11 An attribute which appeared in the parent at a certain time of life tends to appear in the offspring at a corresponding time of life.

The successive stages in the development of every organism present abundant instances of this rule. The embryo resembles the embryo of the parent at a corresponding age, and the successive characters assumed at successive stages appear at the same age in the new being as in the old. Thus the caterpillar emerges from the egg, undergoes repeated moults, changes into a chrysalis, and then into a moth ; and each of these changes occurs at an age corresponding with that at which it appeared in the parents.

Similarly the youth finds his voice breaking and his moustache budding at about the same age at which the same changes took place in his father; and later on he grows stout, his hair turns grey, his skin becomes wrinkled, and his gait shambling at ages corresponding with those in which the same changes appeared in his parents.
This rule is true also of insanity, and many cases have been recorded. Piorry tells of a family every member of which became insane at the age of forty.
Esquirol relates a case in which the grandfather, father, and son all committed suicide when in or near their fiftieth year. Dr. Savage says he has known several instances in which the family inheritance was a tendency to pass into weak-mindedness with melancholy at a certain period of life. This principle may evidently account for some instances of absence of insanity in the children of insane persons. They may not arrive at the time of life at which the insanity would have occurred.


5.24 The operation of reversion sometimes produces curious results. Occasionally, under certain conditions, an individual of one sex will assume many of the characters of the opposite sex, the reversion being to its ancestors of this sex.

... not only structural peculiarities, but habits... and mental qualities, such as courage, are among the characters which may be lost and gained in this manner.

The influence of this form of reversion, which also occurs in the human race, upon the production of insanity, is shown in two ways. In the first place such changes are attended by, and are evidence of, an internal revolution in the general organisation analogous to that which takes place at puberty, when the secondary characters of sex are first assumed. The inversion of sex cannot take place without a commotion analogous to that of the assumption of sex ; and this change takes its place, therefore, among the stresses which will hereafter be considered as determining causes of insanity.

In the second place, this peculiar form of reversion is sometimes accompanied by the production of long-lost characters peculiar to some distant ancestral form. Such a reversion, if it take place in a man or woman, and if it be to some cast of mind and habits peculiar to an ancestor sufficiently remote, that is, to a form of life adapted to widely different surroundings, may itself constitute insanity in the individual in whom it appears.

... when a woman at her climacteric assumes, as some women do, the beard, the diminished mammae and the deep voice and other characters of a man, she may at the same time revert as to habits and mental qualities to some remote feral or semi-feral ancestor of man, and may in consequence exhibit such inability to adapt herself to civilized surroundings as constitutes actual insanity. There is in every asylum a certain number of bearded and bass-voiced women, whose insanity is usually of very intractable type ; and I have had under care at the same time, two men, whose hairless faces, large mammae and shrill voices betokened an assumption of the secondary characters of the other sex, and whose insanity was notably intractable.

5.25 In connection with the subject of reversion falls to be considered the remarkable peculiarity of heredity called prepotence. Such attributes as are common to both parents, and are alike in both, will tend to be accurately reproduced in the offspring ; but where the parents possess contrary or contradictory attributes, the offspring cannot inherit from both, and among these qualities there will be, as it were, a struggle for preponderance, for possession of, or precedence in the offspring. The quality which obtains the mastery, .and succeeds in reappearing in the offspring, is termed prepotent over the other.

Cases frequently occur in which the tendency of a certain quality to appear in the offspring is very strong ; and in such cases this tendency is hereditary, and the quality is transmitted with certainty through many generations. This tendency to " breed true " is an instance of the prepotency of the quality in question.

Like the other manifestations and modifications of the first law of heredity, this of prepotence appears to be very capricious in its application. Some qualities, such as certain colours, are strongly prepotent in some animals, and not at all in others, or may be prepotently transmitted by one sex and not by the other. Occasionally the characters first imported into a race by a single ancestor will appear in generation after generation of that race with ineradicable persistency.
... Napoleonic features... have been transmitted... from some common ancestor of Napoleon Buonaparte and his brothers, to at least the fourth generation... the marriage, in each generation, of the men with women of a different stock, has been insufficient to disturb the powerful prepotent tendency of the features of the father to reappear in the offspring.

5.26 The causes of prepotency are obscure, there being but one circumstance which can be pointed to as of unequivocal influence in enforcing it ; and this is the union of a parent, in whom a quality is present and strongly marked, with one in whom the same quality is latent ; in whom, that is to say, there is an hereditary tendency to assume that quality, but without actual assumption of it.

"... All pigeons have a tendency to become slaty blue, with certain characteristic marks, and it is known that, when a bird thus coloured is crossed with one of any other colour, it is most difficult afterwards to eradicate the blue tint."
5.27 If we consider this question of prepotency in connection with insanity, we shall be able to discover two very valuable rules for practical guidance.

In the first place, if it appears that insanity has become prepotent in a family, then the most stringent measures ought to be adopted to prevent the marriage of the members of that family, and to avoid the transmission of so terrible an inheritance.

In the second place, if there is in a family a tendency to insanity, without that tendency having the force and certainty of prepotence, then, while not interdicting the marriage of its members, the greatest precautions should be taken that they do not marry into families in which a similar tendency exists, for if they do, it is probable that insanity will appear in their offspring.



The Second Law of Heredity.

6.1 The second law of heredity, which I have called the Law of Sanguinity, is likewise an important factor in the production of insanity, and requires the more notice here, since, so far as I know, it has not hitherto received any attention in this regard. The following is the best expression of the law that I have been able to construct:-

There are certain limits, on the one hand of similarity, and on the other of dissimilarity, between two individuals, between which limits only can the union of those individuals be fertile ; and in proportion as these limits are approached, the offspring deteriorates.

Put in a somewhat less accurate but more intelligible form, the law will run thus : There is a certain degree of dissimilarity (sanguinity) between parents, which is most favourable for the production of well-organised offspring ; and parents who are more similar (consanguine), or more dissimilar (exsanguine), will have offspring (if any) whose organisation will be inferior in proportion to the distance of the parents from the most favourable point.


11.4 It has been shown in an earlier part of the book that the amount of intelligence that a person displays depends upon the degree of development of his higher nerve regions. It has been shown how, according as the process of development starts with a vigorous or a feeble impulse, it is carried forward to a high degree of elaboration, or becomes exhausted when a comparatively low level of organisation is reached.

The highest nerve regions, being the last and highest effort of the process of development, the flower of the unfolding organism, are the parts which are most sensitive to any defect in the vigour of the developmental process. If any part fails to reach its full completion owing to the premature exhaustion of the developmental forces, that part will be the highest nerve region. If any part becomes erroneously developed, owing to a bias or fault in the developmental process, that part will be the highest nerve region.

These, we have seen, are the two prime developmental factors in the production of insanity - deficiency and error in the process of development - and the first great division of the forms of insanity rests upon this difference in the developmental cause.

11.5 Where the process of development has come to a premature end, and the higher nerve regions have never attained to the average development of the race, there exists congenital mental deficiency, the dementia naturalis or fatuity a nativitate of jurists. According to the degree in which mental capacity is deficient, this defect is styled by alienists weakness of mind, imbecility, or, in its most marked degree, idiocy ; but the latter term is made to include all degrees, both in law and by non-specialists generally.

11.6 Where the process of development has proceeded to the average extent, so that the individual has attained an average degree of intelligence ; but yet has become defective in its later stages, so that a proneness to insanity is left, which tendency has been made actual by the incidence of some stress ; in such cases there is insanity proper, the dementia accidentalis vel adventitia of jurists.

11.7 Thus it will be seen that in idiocy and weakness of mind, the process of development has not been carried far enough ; while in insanity, the process has been carried far enough, but has diverged into the wrong direction.

11.8 In weakness of mind there is every degree. At one end of the scale is the person who is not quite up to the average ; who is found by his friends to be a little dull ; who was in a lower class at school than others of his age ; who, in spite of assiduous study, got plucked at his examinations ; who is slow to appreciate humour ; who is incapable of entertaining ideas of a moderate degree of abstractness or complexity ; who, if he reads novels, reads those only which deal with incidents and adventures ; who, if he admires pictures, selects those which portray a definite act. Such men, if they possess industry and power of application, often attain a degree of success in life which surprises those who know the narrowness of their intelligence ; the fact being that their interests are so circumscribed that their application to the object they have in view is not apt to stray ; and the attainment of an end depends more on steady, continuous application, than on brilliance of mental ability.

11.9 Beneath this class come those who are recognized as being definitely "deficient"; as being not merely below the average, but below the normal; as being more or less imbecile. The line that divides the dull or weak- minded man from the imbecile is the ability to earn a living. A man who can earn his own living, whose services to the community are of sufficient value to enable him to maintain the standard of living proper to his station in life, may be a dull man, a stupid man, a man of feeble, limited intellect, but he cannot be called an imbecile. Not until his intellectual defect is so grave, that by reason of it he is unable to earn his livelihood, does it become imbecility.

11.10 The distinction between imbecility and idiocy is less clearly marked. All agree that the latter is a more intense degree of the same defect as the former, but there is no precise criterion by which the one can be distinguished from the other. An old legal definition, as quoted by Butknill and Tuke, does indeed say,

"He that shall be said to be a sot and idiot from his birth is such a person who cannot count or number twenty pence, nor tell who was his father or mother, nor how old he is, so as it may appear he hath no understanding or reason what shall be for his profit, or what for his loss ; but if he have sufficient understanding to know and understand his letters, and to read by teaching and information, then it seems he is not an idiot."

It is doubtful how far this definition would be accepted at the present day. There is an old proverb which, so far from regarding as idiotic a person who cannot do so, regards as endowed with exceptional wisdom the child who knows his own father; and if the inability to tell his own age is to render a person liable to be considered idiotic, I know of at least one person who is unable to withstand that test, and is yet considered of average intelligence by his acquaintances. A distinction between imbecility and idiocy may, however, be drawn, and that which I shall propose has the additional advantage of proceeding upon the same lines and belonging to the same system as that already drawn between imbecility and mere weakness of mind.


11.12 ... when an individual fails to reach the full development of the race, the failure will be first noticeable in the activities that are left over after the livelihood has been gained; hence we find that in the first degree of weakness of mind the individual is able to earn his own livelihood, but that when this is done his energies are exhausted. He does not shine socially, he has no "resources," no hobby, no employment for his leisure time. Wife and children he may have, but he takes no part in the education or bringing up of the latter - he lets them find their own way about.

11.13 In the next degree of weakness of mind - in imbecility - the standard of activity has sunk one degree lower. The imbecile is unable to earn his own livelihood. That is the test and that is the criterion of imbecility. He may be capable of doing odd jobs, and of executing simple work under supervision, but his services have not sufficient market value to bring him in enough to support life. He cannot, unassisted, adapt himself to what I have termed his Vital Environment. His activities are not sufficiently developed, and this degree of deficiency of development is imbecility.

11.14 In idiocy the deficiency is still greater. The imbecile fails to adapt himself to his Vital Environment, he fails to complete the second step in his intellectual development; but he surmounts completely the first step, that which enables him to adapt himself to his physical environment. He can be trusted to go out by himself without running the risk of being knocked down by passing vehicles. He can be trusted to cut his own food without cutting his fingers. But the idiot fails to effect even these simple adjustments to his circumstances. He is not only incapable of earning his own living - of adapting himself to his Vital Environment - but he is incapable of preserving himself from the risks of physical harm that are present in his ordinary Physical Environment. So incapable is he of conserving himself against ordinary risks that he cannot be left alone. Although of adult age, he has not proceeded further along the path of development than a young child, and like the young child he requires constant attention. If left by himself he will set himself on fire, or fall into the water, or cut himself, or get entangled in a machine, or come to some actual physical harm which could have been avoided by the exercise of rudimentary intelligence. He fails to adapt himself to the simplest of all the sets of circumstances with which he has to deal, and this extreme degree of failure constitutes idiocy.

11.15 If the doctrine here advanced is true, and if the idiot differs from the person of average intelligence in the fact that the former has not proceeded so far along the path of development as the latter, then, in addition to the evidence of function, we shall expect to find evidence in the structure of the idiot showing that development has stopped short before reaching completion. We shall not expect to find evidence of defect in the early stages of development, for the impetus gained at conception was sufficient to carry the organism through its early stages successfully; but, as the latest stages were not reached, we shall expect to find that the latest acquired structures are wanting.

It has already been explained that the latest acquired structures - the flower, as it were, of the animal organism - are the highest nerve regions ; and it is the want of these regions which constitutes the main physical defect in idiocy.

Side view of right cerebral hemisphere.

A. normal adult

B. adult idiot

C. new-born child

Now the highest nerve regions are situated in the convolutions of the brain, and hence when these regions are greatly wanting in development we shall expect to find:

(1) That the convolutions of the brain are less bulky, contain a less amount of material than normal;

(2) That the convolutions are less elaborate and less complicated in structure than normal; and

(3) That the adult brain which has failed to take on the later stages of development, resembles the brain of an infant, to whom the period for assuming these later stages of development has not yet arrived.

A comparison of the figures... will show how far these reasonings are borne out by the facts.

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a person of normal and average nervous constitution will not be driven mad

stability or instability... nervous arrangements depends... chiefly upon inheritance

the most stringent measures... to prevent the marriage of the members of that family

in idiocy... development has not been carried far enough ; while in insanity, the process has... diverged into the wrong direction.

when an individual fails to reach the full development of the race