Comparative studies have revealed a wealth of commonalities between humans and nonhuman animals that allows them to engage in interspecific social relationships ^{[1][2][3][4][5][6][7]}[14,18,19,20,21,22,23].

As pointed out in ^[1][14], one reason why humans are both willing and capable to relate to animals is the presence of basic biological structures and mechanisms that are relevant in social contexts; these mechanisms are shared between humans and other animals and are highly conserved among vertebrates. Basic mechanisms that enable human intraspecific close relationships and bonds appear to also be involved in theour relationships and bonds with animals, and various nonhuman species may form intense and durable bonds with people (e.g., ^[8][9][24,25]).

Studies on companion and farm animals show that the way in which animals are considered, treated and cared for is strongly affected by peoples’ characteristics such as personality, attitudes, empathy and attachment levels and beliefs in animals’ mental capacities (e.g., ^[10][11][26,27]). In addition, sociodemographic variables, such as gender, age, family structure, education level and previous experiences with animals, play a role ^{[12][13][14][15][16][17]}[8,12,13,28,29,30].

Gender differences are well documented in the literature, with women consistently showing higher levels of empathy and concern regarding animal suffering, holding more positive attitudes towards animals and being more engaged in animal protection and less prone to animal exploitation, animal abuse and cruelty ^{[14][16][18][19][20][21]}[13,29,31,32,33,34]. Apostol et al. ^[14][13], for example, reported that gender was a good predictor of attitudes towards animals, together with empathy towards animals, anthropomorphic beliefs and owning a companion animal.

Personality has been associated with positive attitudes to animals, and there is a relationship between psychopathic personality traits and animal abuse and violence ^[10][22][23][26,35,36]. Empathy and attachment are both related to the quality of human–animal relationships and problems in empathy, attachment and emotion regulation are associated with animal abuse and cruelty ^{[24][25][26][27]}[37,38,39,40].

Attitudes, broadly defined as psychological tendencies to evaluate a particular entity (e.g., humans or animals) with some degree of favor or disfavor ^[28][41], are important in shaping the human–animal relationship and bond and are reported to play a key role in determining animals’ health and welfare. According to ^[13][12], two main aspects underlie human attitudes toward animals: “affect”, which can be defined as people’s affective and emotional responses to animals, and “utility”, i.e., people’s perceptions of animals’ instrumental value. Serpell ^[13][12] suggests that the relative strength of these factors would depend on individual characteristics, experience, cultural factors and also on the specific attributes of animals.

The role of experience and culture in human–animal relationships is well documented in the literature: on the one hand the range of animals kept as pets around the world is exceptionally wide ^[29][42]; on the other hand, human–companion animal relationship styles vary considerably across cultures ^[30][31][7,16]. In Western countries, for example, dogs and cats are mainly kept for companionship, and the idea of eating them is considered intolerable and morally unacceptable; however, in other countries (e.g., China and Vietnam), these species are both kept as pets and consumed ^[32][33][34][43,44,45]. Moreover, in many Western countries, cats and dogs are the most popular and beloved companion animals, but, at the same time, a huge number of them (and other pets) are abandoned, neglected, abused and needlessly euthanized every year ^[35][36][37][46,47,48]. Finally, most people care about pet welfare, but for various reasons, there is still less concern regarding farm animals’ (e.g., pigs, cows and chickens) welfare ^[38][39][49,50].

Regarding animal characteristics, people generally do not see all animals as equal, as their physical and behavioral traits play a role in how they are perceived, considered and treated ^[13][40][41][12,17,51]. Humans tend to prefer animals that are phylogenetically close to them and perceived as physically, behaviorally or cognitively similar; these aspects trigger more positive affect and attachment and caregiving behaviors, as well as greater empathy and a higher concern in terms of welfare and conservation ^{[42][43][44][45][46][47]}[52,53,54,55,56,57]. At the same time, in all human societies, animals are ranked on a ‘‘ladder of worth” as is almost everything else, including other humans ^[48][49][58,59].

Knight et al. ^[41][51] suggested that the variability in people’s attitudes to the use and exploitation of animals depends on a combination of different factors including beliefs about the mental capacities of animals, perceived superiority of humans, availability of alternatives to the use of animals for various purposes (e.g., medical research and food) and whether the problem of animal exploitation has any direct personal relevance. Belief in the animal mind appears to be a good predictor of attitudes towards animals and their use and abuse for the benefit of humans (e.g., entertainment, experimentation and financial gain ^[50][51][60,61]). The propensity to use animals is greater when people believe there is no alternative, when their knowledge of how animals are used is poor, when the affinity with animals is low and when the perceived benefits of using animals outweigh the costs ^[41][51].

Finally, there is evidence that people, when facing situations of conflict regarding animal well-being and suffering, tend to “build arguments” that justify and corroborate their existing attitudes or behavior to avoid dissonance ^[52][62]. This has been clearly demonstrated in the “meat paradox”, which shows that those that consume meat may overcome the cognitive dissonance resulting from a positive attitude towards both animals and meat by living either in a state of “tacit denial” regarding animals being killed to produce meat or by denying that animals can suffer ^[53][54][63,64]. In other words, mental abilities and the capacity for suffering tend to be attributed by people to animals when it is in their interest and motivation and not when it does not suit them ^[55][65].

Given the array of factors involved in most human–animal relationships, it is not surprising that people relate to animals in very different ways that range from highly positive, affectionate and caring to highly negative, dysfunctional or abusive ^{[30][32][48][49][56][57]}[7,43,58,59,66,67]. As Ascione and Shapiro ^[58][68] pointed out, for every study showing that the human–animal relationship can be beneficial and built on love and caring, another deals with animal exploitation or abuse including the abandonment and neglect of companion animals or cases of dog fighting or animal hoarding. Moreover, attachment, empathy and concern for animals do not necessarily guarantee their welfare, and people may disagree on the proper way to treat animals or on what constitutes a fair human–animal relationship ^[12][8].

For example, Mota-Rojas et al. ^[59][69] outlined how adverse consequences on pets’ welfare might depend on widespread and apparently affectionate and caring behaviors, such as dressing pets, application of cosmetics, letting them sleep in beds or overfeeding them, emphasizing that people’s behavior towards companion animals should be based on the understanding and respect of their natural needs rather than on supposed similarities and an affective involvement.

Therefore, people’s relationships with animals in general (farm, zoo and wild animals) and with companion animals cannot be easily categorized into positive (i.e., caring and affectionate) and negative (i.e., neglecting or abusive).

A large body of scientific literature has considered the physical and psychological benefits that both humans and animals may obtain by living together in a reciprocated interaction (e.g., humans: 21, ^{[60][61][62][63][64]}[70,71,72,73,74]; see ^[65][66][75,76] for critical analyses; animals: ^[67][68][69][77,78,79]). However, there is also extensive literature showing that human–animal relationships are characterized by different forms and levels of discomfort and suffering for animals (e.g., ^{[58][59][70][71][72][73]}[11,68,69,80,81,82]).

As occurs in interpersonal relationships, human beings not only and not always care about the well-being of animals but also pursue their own personal needs, desires and goals; human’s psychological characteristics, self-interest and specific contingent needs and goals may, deliberately or not, prevail in jeopardizing the relationship and the animal’s well-being, leading to dysfunctional or even pathological interactions. Keeping wild species under unnatural conditions for leisure or personal gratification, exploiting farm animals through intensive farming causing them high levels of distress and pain, and abandoning domestic animals mostly for trivial reasons are only a few examples of poor and dysfunctional relationships that are topics of research and debate in the human–animal relationship literature ^{[36][74][75][76]}[47,83,84,85].

The type of companionship sought by people may vary to a great degree, and companion animals may be acquired and kept mainly to fulfill different human needs and desires ^{[12][77][78][79][80][81]}[8,86,87,88,89,90]. Some species, either unusual, rare or expensive, are kept as status symbols ^[78][82][83][87,91,92]; some breeds may serve as status objects and are appreciated just for their pedigree or appearance ^[84][93]. Companion animals, mainly dogs and cats, can serve as child substitutes or as toys ^[82][85][86][91,94,95].

Tuan ^[85][94] suggested that when pets are used as toys, they are treated capriciously to gain a sense of power and control that is also expressed through training them to obey to commands. Companion animals, especially dogs, can also serve as extensions of their owners’ self: they may extend their owners’ self not only symbolically by helping them to be something desired but also literally by providing them opportunities to do things that they could not otherwise do such as to engage in childlike games and playful activities or to extend their sphere of interpersonal relationships ^[79][87][88,96]. Keeping pets as an extension of the self implies that they are seen as expressions of the individual’s identity and as part of the person, and thus remaining without them is not conceivable; seeing animals as part of the self involves having an emotional attachment towards them and not just a functional one ^[88][89][97,98].

Finally, there are several examples of negative effects of the human–animal relationship, which include cruel acts and violence towards animals, with various characteristics and different degrees of severity (e.g., ^{[56][72][90][91][92]}[66,81,99,100,101]).

Considering the human–animal relationship with all its nuances helps not only to gain a better understanding of the multidimensional and even contradictory nature of theour interactions with other species but also to further explore the mechanisms involved in the “hows” and the “whys” of human behavior’’ ^{[12][30][56][93]}[7,8,66,102].

Animal hoarding is a highly dysfunctional and pathological form of human–animal relationship, which has been defined by Patronek ^[70][11] as “the third dimension of animal abuse”, since it entails substantial and protracted animal maltreatment and suffering. Due to the fact of its characteristics, it cannot be easily incorporated into the two well-recognized categories of animal cruelty: deliberate animal abuse and neglect ^[70][11]. Indeed, in animal hoarding, animals are exposed to considerable physical and psychological suffering, but often there is a strong human–animal bond, with considerable impairment also of the hoarder’s welfare, who may lack insight regarding the real situation.

Since early reports ^[94][95][177,255], it has clearly emerged that the association between severe animal suffering and a strong attachment to animals was in contradiction with the existing evidence on the human–animal bond ^[70][96][97][11,213,256]. The evidence that has emerged so far invites scholars to an in-depth reflection on the boundaries between the “normal” and pathological aspects of the human–animal relationship and the mechanisms involved ^[12][90][8,99]. For a long time, hoarding animals was considered a “lifestyle” typical of bizarre and strange animal lovers, mainly women, but now it is considered a form of animal abuse, and in recent years, it has been recognized as a mental disorder acknowledged in the DSM-5 as a form of hoarding disorder (i.e., animal hoarding disorder (AHD) ^[98][257], although with its own characteristics and peculiarities.

Hoarding disorder (HD), or compulsive hoarding, is characterized at the behavioral level by problematic behaviors of accumulation, a persistent and considerable difficulty in discarding ordinary items, squalor, personal neglect and poor insight into such disruptive behavior. Hoarding behavior prevents the ordinary use of living spaces in the home, causing significant distress and impairing everyday functioning ^{[98][99][100]}[257,258,259]. HD has been associated to various factors such as interpersonal conflicts, health issues ^[101][260], anxiety-based disorders, depression, family and social disabilities, progressive functional deficit ^[102][261], social isolation and difficulties in bonding with other people ^[103][262]. In addition, an association with cognitive deficits in attention, memory and executive functions (e.g., planning, decision-making and inhibitory control) has been reported ^{[104][105][106]}[263,264,265]. Finally, compulsive hoarding is characterized by a problematic emotional attachment to possessions ^{[99][107][108]}[258,266,267]. According to a psychological cognitive model, this emotional attachment has three specific aspects: possessions provide comfort and security, possessions have human-like qualities and possessions represent an extension of self-concept ^[107][109][266,268].

Animal hoarding (AHD), in its great complexity, appears to have common characteristics with object hoarding including possible underlying cognitive impairments ^[106][265], anxiety, depression, social isolation and relational difficulties ^[110][111][269,270]. However, there is a growing consensus that animal hoarding differs in several respects from object hoarding, the most striking being probably that animal hoarders accumulate living, sentient beings that, differently from objects, need interaction and attention and require continuous nurturing and care. Animal hoarding appears to entail distortions and dysfunctions in attachment, empathy and anthropomorphism and shows how characteristics and motivations that, in general, foster and maintain long-lasting positive relationships with animals may jeopardize the human–animal relationship, causing suffering for both animals and humans ^{[90][112][113]}[99,178,235].

Animal hoarding occurs when an individual persistently accumulates an unusually large number of animals and fails to provide them with the minimum standards of nutrition, hygiene and veterinary care, exposing them to psychological and physical suffering due to the fact of a seriously distorted relationship with them ^[90][114][99,271].

Hoarders often fail to recognize and act on the deteriorated conditions of the animals (e.g., disease, starvation and death), the severe overcrowding, the lack of hygiene of the home environment, and they are often unaware of the negative effects that the hoarding of animals has their own well-being and on that of others living close to them ^{[94][115][116][117]}[177,272,273,274]. Squalid living conditions are common including extreme soiling, parasites, litter, precarious waste, accumulation of feces and urine, nonfunctioning bathrooms and other living spaces, and even dead animals left where they died or are stored ^[70][118][11,275]. Thus, animal hoarding is a multifaceted problem that includes animal maltreatment and abuse, problems related to the health and mental health of hoarders, safety and social and occupational functioning ^[94][95][177,255].

It is worth noting that what defines the disorder is not just the number of animals but the inability of the owner to provide the minimum necessary care for them, to recognize their suffering and to provide them with appropriate sanitary conditions ^{[94][113][119][120][121]}[177,235,276,277,278]. Animal hoarders’ difficulty in relinquishing animals to people who can more adequately care for them is another key point: despite highly negative conditions they form excessive attachments to their animals, consider them like children ^[122][279] and feel the urge to save and care for them, even if this results in significant impairment. They also exhibit intense distress when the animals are removed from their care ^{[107][113][118]}[235,266,275].

Despite its complexity and the highly negative impact on both animals and people, animal hoarding has long been underestimated both within and outside the academic community; however, recently, interest has grown in different research areas such as mental health, psychiatric disorders, human–animal relationships and veterinary practice ^{[90][115][123][124][125]}[99,272,280,281,282].

Animal hoarding is not limited to a specific culture or country, and in addition to the US, it has been reported in various countries, including the UK ^[126][283], Canada ^[121][127][278,284], Australia ^[117][128][274,285], Serbia ^[129][286], Spain ^[116][273], Italy ^[124][281], and Brazil ^[119][130][276,287], and can represent a significant problem ^{[106][115][131]}[265,272,288]. For example, ^[131][288] evaluated the situation of animal hoarding in Germany, reporting 120 animal hoarding cases between 2012 and 2015 for a total of 9174 hoarded animals, mainly cats, dogs and small mammals. The highest number of accumulated animals was documented in ^[124][281] in Italy, with a total of 450 hoarded animals.

Some reviewsearch of the literature provide a portrait of the main characteristics of animal hoarders and the factors that may be involved in the emergence of the disorder ^{[90][125][132]}[99,282,289]. Overall, the studies outline that animal hoarding is a chronic disorder that progressively deteriorates ^[116][117][273,274]. As regards an animal hoarder’s characteristics, the literature shows that the disorder is more frequent among women, who constitute, on average, between 70% and 83% of animal hoarders ^{[90][125][132]}[99,282,289]. This rate has been confirmed in most of the reported hoarding cases (e.g., ^{[106][116][128][133]}[265,273,285,290]). Although the reasons for this gender effect have not yet been ascertained, it could be related to the greater predisposition of women to empathize with animals and to be attracted by the infantile characteristics typical of most pets, especially dogs and cats, which are in fact the species most involved in hoarding situations. Gender differences in attitudes, empathy and attachment and concern towards animals have also been well documented in the literature on the “bright side” of the human–animal bond ^[14][16][13,29].

Individuals showing animal hoarding are reported to be socially isolated, in their fifties or sixties, on average, when identified ^{[106][116][117][119][128]}[265,273,274,276,285] and in approximately 70% of the cases are single, divorced or widowed ^[106][119][265,276]. Studies also show that hoarders are often unemployed or retired but may also be breeders who initially bred animals for economic reasons ^{[117][119][128][133][134]}[274,276,285,290,291]. However, the phenomenon emerges in all demographic socioeconomic conditions ^{[90][125][132]}[99,282,289]; even individuals who are well integrated into society may be affected (e.g., health care workers, public employees, lawyers and veterinarians), and not uncommonly hoarders live with people who depend on them, including children, people with disabilities or elderly people, who may find themselves sharing the same living conditions as the animals being accumulated.

Three main types of animal hoarders have been described in the literature, who show different characteristics and different levels of severity ^[94][135][177,292]. These characteristics regard the presence of medical/psychological problems, involvement in society, awareness of the problem, attitude towards authority and risks for animals but also attachment to animals, empathy towards them and the tendency to anthropomorphize them ^[90][113][99,235].

The “overwhelmed caregivers” ^[94][135][177,292] are usually lonely people with a strong attachment to and empathy for animals, who initially provide proper care to them but eventually become overwhelmed due to the fact of difficulties such as illness, economic problems or a bereavement. Their self-esteem is linked to the role of caregiver, and they often adopt animals in a passive way (people give them animals knowing they love them). These hoarders may have mood disorders but show a certain level of awareness of their problems, respect authority and are cooperative.

For “rescue hoarders” ^[94][135][177,292], saving animals is a mission and they are convinced they are the only ones who can provide adequate care to them. These hoarders have a need to acquire animals actively, adopting them from shelters or through flyers or social networks. After rescuing animals, these hoarders prevent their adoption, are afraid they could die and are opposed to euthanasia even when seeing them suffering and healing them is impossible. Thus, the number of animals gradually overwhelms their capacity to provide the minimal care. They are not necessarily isolated but may have a network of helpers that provide them animals to care for. Indeed, these hoarders may be found among people working in rescue shelters or in veterinary clinics, who believe they can save all animals by taking them home ^[134][291]. These types of hoarders avoids authorities and/or impedes their access, making the solution to the problem difficult.

The last type reported in the literature is the “exploiter hoarders” ^[94][135][177,292], who acquires animals to serve their own needs (e.g., absolute control and demonstration of expertise). They lack empathy for people/animals, are indifferent to the harm caused to animals or people and are frequently manipulative and devise strategies to avoid controls. They tend to deny the problem and reject concerns from any authorities over the animals’ care. However, it has been outlined that exploitative hoarding is often associated with sociopathic traits or personality disorders, either narcissistic or antisocial, that resemble much more those of people who engage in criminal behavior and animal abuse and cruelty.

Overall, the literature indicates that a hoarder’s story begins with relatively few animals, and that gradually, the situation deteriorates through the continual acquisition of animals (actively or passively) despite a lack of money and time for them, avoidance of sterilization, insufficient veterinary care and the struggle to keep the house sufficiently clean. The incipient hoarder and the breeder–hoarder are considered intermediate stages that may evolve over time into full-blown hoarding ^[135][136][292,293]. Different from an incipient hoarder, a breeder initially keeps animals for shows or to sell but over time finds it increasingly difficult to care for them properly. They do not always keep the animals in their own home; thus, their living conditions may not always be as neglected as those of the animals. However, this is not the rule, since many hoarders set-up home breeding. They usually have only a moderate awareness of the state of the animals and their ability to care for them; thus, they continue to raise them.

In general, animal hoarders show a highly dysfunctional relationship with animals, characterized by distorted overattachment to them, empathy disfunction and a high level of anthropomorphism ^{[90][113][132]}[99,235,289]. Their behavior, especially that of incipient hoarders, overwhelmed caregivers and rescue hoarders, is most often rooted in a real desire to take care of animals or save them and is characterized by a strong emotional attachment to animals and an extreme distress, often genuine despair, at the idea of being separated from them. Patronek and Nathanson ^[137][294] reported that when attempting to account for their behavior, these hoarders often mention their love for animals. However, the situation degenerates from helping into a form of abuse such that, even though the intent is not to harm animals, severe and prolonged suffering is caused to them ^[118][275].

In most hoarding situations, the emotional attachment to animals, which is a basic aspect of the human–animal relationship, is distorted, formed with many individuals and immediately triggered to the point that any animal encountered can be easily seen as one’s own and the individual feels obliged to take care of it ^[138][295].

It could be hypothesized that empathy (in particular affective empathy) could be the initial mechanism that makes these people sensitive to the needs and suffering of animals, motivating them to take care of them; however, when facing the evidence of their inability to care for animals properly, a denial process would take place to protect their sense of identity and self-esteem, both strongly rooted in the link with animals, in the belief of a special connection with them and in the presumed ability to take care of them. Furthermore, empathy could decrease as a result of exposure to suffering due to the fact of habituation and/or to avoid an excessive level of personal distress or even to dealing with many individuals and a reduction in personal contact ^[139][140][134,135]. Conversely, “exploiter hoarders” generally lack empathy for animals and people, being more similar to people with an antisocial personality and who exhibit such behavior and are prone to animal abuse, in whom empathy towards animals and people is compromised in some way ^[141][142][118,296].

Hoarders also tend to anthropomorphize animals to a greater extent compared to ordinary animal owners. Steketee et al. ^[113][235] observed that 81% of animal hoarders (compared to 27% of owners) tended to ascribe to them the same characteristics and intelligence of humans and to view them as their “children”. In fact, they often report a strong attachment to their animals and consider them to be like children ^[122][279]. This often results in a distorted sense of responsibility and a strong need for control over the animals, whereby the hoarders feel that they must acquire animals and should not separate from them to ensure that nothing bad happens to them.

In “rescuer hoarders”, these aspects would be strongly linked to the problem of death: they appear to consider the deterioration of their living conditions as a necessary sacrifice to help creatures in need, who might otherwise die ^[138][295], and some of them explicitly state that they would like to create shelters that do not include euthanasia ^[143][297]. Thus, there is a real urge to rescue animals, which is experienced as a duty, leads to a strong sense of guilt if disregarded and is linked to the constant concern that something terrible could occur to the animals if they were not helped (e.g., being hit by a car or ending up in a vivisection laboratory) ^[138][295]. Since animals, as all living beings do, die and separation from them is inevitable, anxiety relative to control and responsibility is further enhanced in hoarders ^[137][294]. These individuals may have intense emotional reactions of anger or distress triggered by thoughts of loss or separation ^[105][264] and often are unable to separate themselves from the bodies of dead animals, keeping them inside the house ^{[94][95][118]}[177,255,275]. Based on these extreme reactions of separation-related distress and anger, Reference ^[144][298] hypothesized that the propensity of animal hoarders to ignore the problems arising from acquiring an increasing number of animals and to convince themselves that the animals are well might be a way to avoid the unpleasant feelings that would result from giving animals up for adoption or from acknowledging the severely poor conditions of their animals.

Emotional attachment, concern for animals and empathy are less explanatory in the case of “exploiter hoarders”, whose motivations for hoarding could be linked to a need to dominate and control or to financial interest (especially in the case of anti-social personalities) or to the desire to establish relationships that confirm their value, in which other individuals (in this case animals) have the role of self-objects, serving to ensure attentions and adoration (in narcissists; ^[135][292]). Similar motivations, however, can be observed in a far more nuanced form also in so called “normal” non abusive human–animal relationships ^{[12][79][85][145]}[8,88,94,299].

For animal hoarders, animals also have an instrumental role, which is functional to the preservation of their sense of identity and self-esteem, associated with the role of caregiver and is constantly reinforced by the perception of having positive relationships with living beings that are sentient and totally dependent ^[137][138][294,295]. Self-esteem would derive partly from the sense of self-efficacy—through the control over the animals (especially in the exploiter) and in part by seeing recognized their ability as caregivers through the affection received by the animals, ^[137][146][294,300]. However, the primary source of self-esteem is the interaction within the human–animal relationship: animals are highly focused on the person and do not judge or criticize, and they cannot object to misinterpretations of their feelings and needs ^[137][294].

The central role of animals in fostering and maintaining an individual’s self-esteem and sense of identity has also been documented in studies on normal human–animal relationships and are one of the psychologically positive effects of the human–animal bond ^{[61][147][148]}[71,301,302].

Like non-hoarders, incipient hoarders, overwhelmed caregivers and rescue hoarders derive a sense of security and comfort due to the fact of animals’ capacity to provide emotional support and unconditional love in a relationship that is perceived as less dangerous than those with other people ^[118][134][275,291]. Animal hoarders are reported to have difficulties in establishing affective bonds with others, tend to maintain social isolation ^[111][113][235,270] and prefer contact with animals ^[118][134][275,291]. They could consider their relationships with animals as safer and more rewarding than interactions with humans ^[118][137][275,294]. Cats and dogs are the most accumulated species ^[121][132][278,289] but also the two most common species of companion animals; they have a long history of domestication and close association with humans, live close to them and are widely considered as important social partners by their owners in many countries. However, hoarded animals may include a variety of animals, such as miniature ponies, deer, ferrets, pigs, various species of birds, and even spitting llamas, and multiple species may be present in any isolated hoarding case ^[121][278].

Steketee et al. ^[113][235] interviewed individuals who fit the criteria for animal hoarding and individuals owning many animals but not meeting the hoarding criteria, reporting that both hoarders and non-hoarders had stressful life events in childhood and adulthood, strong feelings about animals such as the desire to rescue, take care of and be close to them. Animal hoarders, however, had more dysfunctional interpersonal relationships and mental health concerns and anthropomorphized animals more often; however, the most significant difference between the two samples was the presence of a chaotic domestic environment during childhood and childhood problems with caregivers (e.g., unstable, neglectful, abusive, absent, and/or inconsistent parents). In another study ^[93][102], it was analyzed whether people owning 20 or more cats shared the commonly reported psychological and demographic profile of animal hoarders compared to owners of 1–2 cats drawn from the same population. They found that people who owned many cats were more similar to clinical animal hoarders in age and pet attachment levels than the typical cat owners, but they differed in functioning, veterinary care and home organization.

The quality of caregiving and attachment during infancy seem to play an important role in the emergence of animal hoarding, and animal hoarders often report that during childhood they relied on companion animals, suggesting that in difficult developmental situations companion animals may function as alternative attachment figures providing intimacy and security without fear of rejection ^[149][303]. Nathanson and Patronek ^[150][109] hypothesized that when parenting was neglectful, inconsistent or abusive, animals became crucial in animal hoarders’ childhood to maintain and promote empathy, comfort, calm, acceptance and self-esteem, as previously suggested by Brown ^{[88][89][151]}[97,98,304] in the theory of animals as self-object. Secure attachment in early childhood is considered essential for normal emotional development, emotion regulation, empathy and good interpersonal relationships ^[149][152][205,303] and attachment to pets plays an important role in normal social, emotional and cognitive development, promoting mental health, well-being and quality of life ^[1][5][14,21].

Due to the occurrence of developmental and life events, animal hoarders may develop in adulthood a compensatory over-reliance and an overattachment to animals, with animals becoming a dysfunctional solution to cope with their need for relationships and intimacy without fear of rejection and abandonment ^[137][294]. An abusive, traumatic or dysfunctional childhood is correlated with a disorganized attachment style, which can result in compulsive caregiving. This caregiving style, differently from the sensitive caregiving style (i.e., the ability to be responsive and attuned with another’s individual support-seeking behavior), is characterized by the tendency to provide care obsessively and intrusively, irrespective of whether the care is wanted or needed ^{[153][154][155]}[193,305,306]. In adulthood the compulsive caregiving of animals can become the primary way to maintain or building a sense of self. This kind of controlling behavior often characterizes the caregiving style of animal hoarders, together with other forms of control typical of animal hoarders including refusal to adopt, rejection of help and expert opinions regarding proper animal care and sometimes the saving of dead bodies ^[137][294].

Probably, one of the most perplexing aspects of animal hoarding is that animal hoarders declare to love animals and want to care for them but, in fact, their animals are terribly neglected and suffering. This lack of insight is typical of animal hoarders, and it has been suggested that being unaware of the degradation in their personal lives and those of their animals could be suggestive of dissociation ^[137][294].

Dissociation represents a self-protective strategy to avoid negative feelings associated with distress or trauma ^[156][307] and can make it difficult to understand and respond to others’ feelings as well as easier to view them as less than human ^[149][303]. In the case of animal hoarders, dissociation would represent a strategy to preserve the integrity of the self, the self-image and the mission of caregivers, notwithstanding the extremely precarious conditions of their animals.

It has been argued that dissociation may be best understood as a continuum from more common, normal manifestations to less common and more pathological symptoms ^[157][308]. Indeed, in ^[158][309] it was reported that in a sample of college students, dissociation was positively associated with attachment to companion animals, and this result was then replicated in another student sample ^[159][310].