Helping in cooperatively breeding long-tailed tits: a test of Hamilton's rule

Inclusive fitness theory provides the conceptual framework for our current understanding of social evolution, and empirical studies suggest that kin selection is a critical process in the evolution of animal sociality. A key prediction of inclusive fitness theory is that altruistic behaviour evolves when the costs incurred by an altruist (c) are outweighed by the benefit to the recipient (b), weighted by the relatedness of altruist to recipient (r), i.e. Hamilton''s rule rb > c. Despite its central importance in social evolution theory, there have been relatively few empirical tests of Hamilton''s rule, and hardly any among cooperatively breeding vertebrates, leading some authors to question its utility. Here, we use data from a long-term study of cooperatively breeding long-tailed tits Aegithalos caudatus to examine whether helping behaviour satisfies Hamilton''s condition for the evolution of altruism. We show that helpers are altruistic because they incur survival costs through the provision of alloparental care for offspring. However, they also accrue substantial benefits through increased survival of related breeders and offspring, and despite the low average relatedness of helpers to recipients, these benefits of helping outweigh the costs incurred. We conclude that Hamilton''s rule for the evolution of altruistic helping behaviour is satisfied in this species.     

Investment and relatedness: A cost/benefit analysis of breeding and helping in the pied kingfisher (Ceryle rudis)

Helping at the nest in birds is often termed altruism. However, so far, no study has ever demonstrated high costs to a helper's own lifetime reproductive success (=direct fitness), nor its compensation through benefits from relatives other than its own offspring (=indirect fitness). In this paper on pied kingfishers (Ceryle rudis) the relationship between investment, relatedness and inclusive fitness (expressed in terms of genetic equivalents) is investigated for breeding males, and males that help either relatives (=primary helpers) or strangers (=secondary helpers). With respect to guarding nests against predators and feeding young, primary helpers invest as much as breeders, but secondary helpers contribute significantly less. These differences in status and investment (measured in energy expenditure) affect the birds' future to such an extent that primary helpers have a lower chance of surviving and mating than secondary helpers. However, their costs in direct fitness are compensated by pronounced benefits to indirect fitness, resulting from improved survival of siblings and parents. An attempt is made to calculate the inclusive fitness of birds following different strategies over a 2-year period. It is concluded that (a) breeding is superior to helping and helping superior to doing nothing and (b) that kin-selection must be invoked to explain why surplus males choose the more costly primary helper strategy instead of the cheaper secondary helper strategy. Alternative explanations, including group selection, parental manipulation and reciprocity, are discussed.     

Female‐Biased Helping in a Cooperatively Breeding Bird: Female Benefits or Male Costs?

There is often a sex bias in helping effort in cooperatively breeding species with both male and female helpers, and yet this phenomenon is still poorly understood. Although sex‐biased helping is often assumed to be correlated with sex‐specific benefits, sex‐specific costs could also be responsible for sex‐biased helping. Cooperatively breeding brown jays (Cyanocorax morio) in Monteverde, Costa Rica have helpers of both sexes and dispersal is male‐biased, a rare reversal of the female‐biased dispersal pattern often seen in birds. We quantified helper contributions to nestling care and analyzed whether there was sex‐biased helping and if so, whether it was correlated with known benefits derived via helping. Brown jay helpers provided over 70% of all nestling feedings, but they did not appear to decrease the workload of breeders across the range of observed group sizes. Female helpers fed nestlings and engaged in vigilance at significantly higher levels than male helpers. Nonetheless, female helpers did not appear to gain direct benefits, either through current reproduction or group augmentation, or indirect fitness benefits from helping during the nestling stage. While it is possible that females could be accruing subtle future direct benefits such as breeding experience or alliance formation from helping, future studies should focus on whether the observed sex bias in helping is because males decrease their care relative to females in order to pursue extra‐territorial forays. Explanations for sex‐biased helping in cooperative breeders are proving to be as varied as those proposed for helping behavior in general, suggesting that it will often be necessary to quantify a wide range of benefits and costs when seeking explanations for sex‐biased helping.     

Broodcare helpers in the cichlid fish Lamprologus brichardi: Their costs and benefits

‘Helping’ in birds and mammals involves seemingly altruistic behaviour. In the cichlid fish Lamprologus birchardi helpers are usually young of former broods staying in their parents' territories and participating in all kinds of parental duties (Broodcare, territory maintenance and defence). The discovery of helpers in fish offered the chance of attempting an extensive analysis of potential costs and benefits influencing the evolution of helpers in a vertebrate. Three factors proved to be of major importance in the cost-benefit analysis of helping as opposed to leaving for family-independent nonreproductive aggregations. Due to investment and to their rank within a family's hierarchy, helpers grow at a slower rate than non-helpers. This cost is compensated for by (i) a lower mortality risk to helpers caused by their access to a defended shelter and by protection afforded by bigger family members, and (ii) a positive contribution by helpers to the future reproductive success of their parents: females with helpers produce bigger clutches and consequently more free-swimming fry (=siblings). Other variables, such as the helpers' influence on the relative breeding success of their parents, broodcare experience through helping, the chances of territory take-over, parasitism of parents' reproduction and cannibalism are of minor importance. Similar social organizations in other fish are discussed with respect to their ecology and are compared with cooperatively breeding birds and mammals.     

Genetic analysis reveals diverse kin‐directed routes to helping in the rifleman Acanthisitta chloris

The social organization of cooperatively breeding species is extremely variable, with diverse social group composition and patterns of relatedness. Species that exhibit alternative routes to helping within the same population are potentially useful systems to investigate the causes and fitness consequences of diverse evolutionary pathways to cooperative behaviour. In this study, we use microsatellite markers and field observations to describe helping behaviour and patterns of relatedness in the unusual cooperative breeding system of the rifleman Acanthisitta chloris. First, we show that rifleman helpers consist of a remarkably diverse demographic, including males and females, who may be adult or juvenile, failed breeders or nonbreeders, or even successful breeders that simultaneously feed their own brood. Adult helpers mostly helped at first‐brood nests, while first‐brood juveniles assisted their parents at second broods. Second, we show that rifleman pairs are strictly sexually monogamous, and helpers did not gain any current reproductive success through helping. Third, genotyping showed that contrary to previous assumptions, helpers were closely related to the recipients of their care and preferentially directed care towards relatives over contemporaneous nests of nonrelatives. Finally, we show that variation in helper provisioning effort was attributed to age: juvenile helpers provisioned less than adults and were less responsive to the demands of a growing brood. Overall, our results show that the diverse routes to helping in this unusual species are driven by the common theme of kinship between helper and recipients, resulting in a previously underestimated potential for helpers to gain indirect fitness benefits.     

Caring across Boundaries versus Keeping Boundaries Intact: Links between Moral Values and Interpersonal Orientations

Prior work has established robust diversity in the extent to which different moral values are endorsed. Some people focus on values related to caring and fairness, whereas others assign additional moral weight to ingroup loyalty, respect for authority and established hierarchies, and purity concerns. Five studies explore associations between endorsement of distinct moral values and a suite of interpersonal orientations: Machiavellianism, prosocial resource distribution, Social Dominance Orientation, and reported likelihood of helping and not helping kin and close friends versus acquaintances and neighbors. We found that Machiavellianism (Studies 1, 3, 4, 5) (e.g., amorality, controlling and status-seeking behaviors) and Social Dominance Orientation (Study 4) were negatively associated with caring values, and positively associated with valuation of authority. Those higher in caring values were more likely to choose prosocial resource distributions (Studies 2, 3, 4) and to report reduced likelihood of failing to help kin/close friends or acquaintances (Study 4). Finally, greater likelihood of helping acquaintances was positively associated with all moral values tested except authority values (Study 4). The current work offers a novel approach to characterizing moral values and reveals a striking divergence between two kinds of moral values in particular: caring values and authority values. Caring values were positively linked with prosociality and negatively associated with Machiavellianism, whereas authority values were positively associated with Machiavellianism and Social Dominance Orientation.     

Group-size-dependent punishment of idle subordinates in a cooperative breeder where helpers pay to stay

In cooperative breeding systems, dominant breeders sometimes tolerate unrelated individuals even if they inflict costs on the dominants. According to the ‘pay-to-stay’ hypothesis, (i) subordinates can outweigh these costs by providing help and (ii) dominants should be able to enforce help by punishing subordinates that provide insufficient help. This requires that dominants can monitor helping and can recognize group members individually. In a field experiment, we tested whether cooperatively breeding cichlid Neolamprologus pulcher subordinates increase their help after a forced ‘idle’ period, how other group members respond to a previously idle helper, and how helper behaviour and group responses depend on group size. Previously, idle helpers increased their submissiveness and received more aggression than control helpers, suggesting that punishment occurred to enforce help. Subordinates in small groups increased their help more than those in large groups, despite receiving less aggression. When subordinates were temporarily removed, dominants in small groups were more likely to evict returning subordinates. Our results suggest that only in small groups do helpers face a latent threat of punishment by breeders as predicted by the pay-to-stay hypothesis. In large groups, cognitive constraints may prevent breeders from tracking the behaviour of a large number of helpers.     

Paying to stay or paying to breed? Field evidence for direct benefits of helping behavior in a cooperatively breeding fish

Several hypotheses aim to explain the evolution of helping behavior,but conclusive experimental support for evaluating the relativeimportance of individual hypotheses is still lacking. We reporton two field experiments conducted to test the "territory inheritance"and "pay-to-stay" hypotheses in the cooperatively breeding cichlidfish Neolamprologus pulcher The territory inheritance hypothesiswas tested by removing one parent, which created breeding vacancies.In 39% of cases, same-sex helpers took over the breeding spot;in 44% of cases helpers continued helping new breeders, and17% were evicted by new breeders. Helpers that were closelysize matched to the removed breeder had a better chance of gainingthe breeding spot Male helpers tended to continue helping aftera takeover more often than females.The pay-to-stay hypothesiswas tested-by temporarily removing helpers. Whereas breedersdid not respond aggressively to removals, other group membersattacked the removed helpers on their return, and 29% were eventuallyevicted. The returning helpers assisted more by increasing theirrate of territory maintenance and defense and visiting the broodchamber more frequently Size and sex of removed helpers didnot explain the observed aggressive reactions of other groupmembers. Thus, our results support both hypotheses: N. pulcherneeds to pay with help to be allowed to remain protected inthe family group, and there they may inherit the natal territory.N. pulcher helpers gain direct benefits from helping behavior.     

Extrapair paternity, inclusive fitness, and within-group benefits of helping in western bluebirds

In central coastal California, USA, 3–16% of western bluebird ( Sialia mexicana ) pairs have adult male helpers at the nest. Demographic data on a colour-ringed population over a 13-year period indicate that helpers gain a small indirect fitness benefit through increases in the number of young fledged from nests of close kin. A small proportion of adult helpers (16%) that were able to breed and help simultaneously had higher annual inclusive fitness than males that only bred. These males comprised such a minor proportion of helpers that the mean fitness of helpers was still lower than the mean fitness of independent breeders. We used DNA fingerprinting to determine whether extrapair fertilizations alter within-group benefits enough to tip the balance in favour of helping behaviour. Overall, 19% of 207 offspring were sired by males other than their social father and extrapair fertilizations occurred in 45% of 51 nests. Intraspecific brood parasitism was rare so that mean mother-nestling relatedness approximated the expected value of 0.5. Extrapair paternity reduced putative father-offspring relatedness to 0.38. Mean helper-nestling relatedness was 0.41 for helpers assisting one or both parents and 0.28 for helpers aiding their brothers. Helpers rarely sired offspring in the nests at which they helped. Helping was not conditional on paternity and helpers were not significantly more closely related to offspring in their parents' nests than to offspring in their own nests. Although helpers may derive extracurricular benefits if helping increases their own or their father's opportunities for extrapair fertilizations, within-nest inclusive fitness benefits of helping do not compensate males for failing to breed. Breeding failure and constraints on breeding are the most likely explanations for why most helpers help.     

Sex (similarities and) differences in friendship jealousy

Friendships provide material benefits, bolster health, and may help solve adaptive challenges. However, a recurrent obstacle to sustaining those friendships—and thus enjoying many friendship-mediated fitness benefits—is interference from other people. Friendship jealousy may be well-designed for helping both men and women meet the recurrent, adaptive challenge of retaining friends in the face of such third-party interference. Although we thus expect several sex similarities in the general cognitive architecture of friendship jealousy (e.g., it is attuned to friend value), there are also sex differences in friendship structures and historical functions, which might influence the inputs of friendship jealousy (e.g., the value of any one friendship). If so, we should also expect some sex differences in friendship jealousy. Findings from a reanalysis of previously-published data and a new experiment, including both U.S. student and adult community participants (N = 993), provide initial support for three predicted sex differences: women (versus men) report greater friendship jealousy at the prospective loss of best friends to others, men (versus women) report greater friendship jealousy at the prospective loss of acquaintances to others, and men's (but not women's) friendship jealousy is enhanced in the context of intergroup contests.     

Fitness consequences of helping behavior in the western bluebird

We examined the fitness consequences of helping behavior inthe western bluebird (Sialia mexicana) at Hastings Reservationin Carmel Valley, California, USA, and tested hypotheses forhow helpers benefit from engaging in alloparental behavior.Both juvenile and adult western bluebirds occasionally helpat the nest During a 12 year period, all adult helpers and mostjuvenile helpers were male. Helpers usually fed at nests ofboth their parents and rarely helped when only one parent waspresent. The frequency of pairs with adult helpers was only7%, but nearly one-third of adult males helped among those withboth parents on the study area. At least 28% were breeders whosenests failed. The propensity to help appears to depend uponparental survival, male philopatry, and the breeding successof potential helpers. Feeding rates were not increased at nestswith juvenile helpers, apparendy because breeding males reducedtheir feeding rates. In contrast, adult helpers increased theoverall rates of food delivery to the nest in spite of a reductionin the number of feeding trips made by both male and femaleparents. Helpers did not derive any obvious direct fitness benefitsfrom helping, but they had greater indirect fitness than nonhelpersdue to increases in nestling growth rates and fledging successat their parents' nests. Helpers fledged fewer offspring intheir first nests than did nonhelpers, suggesting that theywere birds with reduced reproductive potential. Although wehave not yet measured the effect of extrapair fertilizationson the fitness benefits of helping, we calculated the differencein fitness between helpers and nonhelpers as a function of thepotential helper's paternity when breeding independently andhis father's paternity in the nest at which he might help. Inconjunction with constraints on breeding and indirect fitnessbenefits, we predict that relatedness of males to the youngin their own as well as their parents' nests will influencehelping behavior in western bluebirds.     

Social organization and communal breeding in the chestnut-bellied starling (Spreo pulcher)

Chestnut-bellied starlings (Spreo pulcher) live in social groups of 10–30 individuals and during their breeding seasons maintain group ranges which show little overlap with neighbouring groups. A social group may contain two to six breeding pairs, non-breeding adults of both sexes, and immatures. Each breeding female has her own nest and she alone incubates. The parents and up to 12 other starlings feed the nestlings. Individual helpers may successively or simultaneously attend the nests of different breeders. The percentage of nests attended within a group differs for helpers of different sex, age and breeding status: immatures of 1–2 years and non-breeding adult males help most and adult females least. Nests with more helpers have higher fledging success than those with fewer helpers. These results are discussed with reference to the tentative benefits of helping behaviour and kinship relationships within the social group.     

Altruism among kin vs. nonkin: effects of cost of help and reciprocal exchange

Evolutionary principles suggest that there will be differences in the nature of altruism directed toward kin vs. nonkin. The present study sought to explore these differences. Participants were 295 undergraduate students who each completed a questionnaire about help exchanged with siblings, cousins, acquaintances or friends. For siblings, cousins and acquaintances, greater relatedness was associated with higher levels of helping. Friends were an exception, however, receiving as much or more help than kin. Consistent with an evolutionary analysis, as the cost of helping increased, kin received a larger share of the help given, whereas nonkin received a smaller share. For low-cost help, people helped friends more than siblings; for medium-cost help, they helped siblings and friends equally; and for high-cost help, they expressed a greater willingness to help siblings than friends. As expected, the level of reciprocal exchange was higher among acquaintances than among friends; however, there was also an unexpectedly high level of reciprocal exchange among kin.     

Helper effects on pup lifetime fitness in the cooperatively breeding red wolf (Canis rufus)

The evolutionary maintenance of cooperative breeding systems is thought to be a function of relative costs and benefits to breeders, helpers and juveniles. Beneficial effects of helpers on early-life survivorship and performance have been established in several species, but lifetime fitness benefits and/or costs of being helped remain unclear, particularly for long-lived species. We tested for effects of helpers on early- and late-life traits in a population of reintroduced red wolves (Canis rufus), while controlling for ecological variables such as home-range size and population density. We found that the presence of helpers in family groups was positively correlated with pup mass and survival at low population density, but negatively correlated with mass/size at high density, with no relation to survival. Interestingly, mass/size differences persisted into adulthood for both sexes. While the presence of helpers did not advance age at first reproduction for pups of either sex, females appeared to garner long-term fitness benefits from helpers through later age at last reproduction, longer reproductive lifespan and a greater number of lifetime reproductive events, which translated to higher lifetime reproductive success. In contrast, males with helpers exhibited diminished lifetime reproductive performance. Our findings suggest that while helper presence may have beneficial short-term effects in some ecological contexts, it may also incur long-term sex-dependent costs with critical ramifications for lifetime fitness.     

Gratefully Received,Gratefully Repaid: The Role of Perceived Fairness in Cooperative Interactions

It is well documented that people would remunerate fair behaviours and penalize unfair behaviours. It is argued that individuals’ reactions following the receipt of a gift depend on the perceived intentions of the donors. Fair intentions should prompt positive affect, like gratitude, triggering cooperative behaviours; while intended unfairness should trigger negative affect, like anger, fostering anti-social actions. It is, however, contended that when people lack information to infer others’ intention they may use ‘normative’ beliefs about fairness - what a typical fair individual ‘should’ do in these circumstances – to guide their behaviour. In this experiment we examined this assertion. We had 122 participants play a one-shot, double-anonymous game with half playing as potential helpers (P1s) and half as recipients (P2s). Whether a participant was a P1 or P2 was chance-determined and all participants knew this. P1s decided whether to help P2s and whether to make their help unconditional (no repayment needed) or conditional (full or ‘taxed’ repayment). P2s decided whether to accept the offer and whatever conditions attached but were blind to the list of helping options available to P1s. We anticipated that recipients would refer to the ‘injunctive norm’ that ‘fair people should help “for free” when it is only by chance that they are in a position to help’. Therefore, without knowing P1s’ different helping options, unconditional offers should be rated by recipients as fairer than conditional offers, and this should be linked to greater gratitude with greater gratitude linked to greater reciprocation. Path analyses confirmed this serial mediation. The results showed that recipients of unconditional offers, compared to conditional ones, interpreted the helpers’ motives as more helpful, experienced greater gratitude and were more eager to reciprocate. The behavioural data further revealed that, when given a latter option to default, 38% of recipients of conditional offers did so.     

Is there an optimal number of helpers in Alpine marmot family groups?

The consequence of helping behavior on breeders fitness is stillcontroversial. We used multivariate analyses to investigatefor the effects of male and female subordinates on breeders'components of fitness in the Alpine marmot, Marmota marmota.We found that male and female subordinates, respectively, increasedand decreased juvenile survival during winter. Thus, we giveevidence that male subordinates should be considered as helpers,and that helpers provided breeders with immediate reproductivesuccess gains, whereas subordinates females were costly. Helpershad no positive effects on female body condition, on persistence(future survival) of dominants, and on future reproduction (occurrenceand size of a litter). Helpers thus did not provide breederswith delayed fitness benefits, and therfore, the load-lighteninghypothesis was not supported. On the contrary, helpers had delayedfitness cost for dominant males and, consequently, for dominantfemales. Immediate benefits counterbalanced by delayed costssuggested an optimal number of helpers in the family group bothfrom male and female perspectives. An optimality model wellpredicted the observed mean number of helpers in Alpine marmotfamily groups. Optimal numbers of helpers were slightly differentfor males and females, suggesting a potential conflict of interestbetween dominants. We finally discuss the possible mechanismsof helping that may explain the observed pattern in the Alpinemarmot.     

Temporal but not spatial environmental variation drives adaptive offspring sex allocation in a plural cooperative breeder

Cooperatively breeding birds have been used frequently to study sex allocation because the adaptive value of the sexes partly depends upon the costs and benefits for parents of receiving help. I examined patterns of directional sex allocation in relation to maternal condition (Trivers-Willard hypothesis), territory quality (helper competition hypothesis), and the number of available helpers (helper repayment hypothesis) in the superb starling, Lamprotornis superbus, a plural cooperative breeder with helpers of both sexes. Superb starlings biased their offspring sex ratio in relation to prebreeding rainfall, which was correlated with maternal condition. Mothers produced relatively more female offspring in wetter years, when they were in better condition, and more male offspring in drier years, when they were in poorer condition. There was no relationship between offspring sex ratio and territory quality or the number of available helpers. Although helping was male biased, females had a greater variance in reproductive success than males. These results are consistent with the Trivers-Willard hypothesis and suggest that although females in most cooperatively breeding species make sex allocation decisions to increase their future direct reproductive success, female superb starlings appear to base this decision on their current body condition to increase their own inclusive fitness.     

Direct fitness benefits of group living in a complex cooperative society of carrion crows, Corvus corone corone

The social behaviour of carrion crows varies between populations. In northern Spain cooperatively breeding groups form through delayed natal dispersal and/or immigration of individuals (usually males) into the territory. In this population, carrion crows therefore breed as either unassisted pairs, pairs with nondispersing 1-2-year-old helpers (nondispersers), pairs with immigrant helpers or mixed groups (pairs with both immigrants and nondispersers). We used a microsatellite-based genotyping system to determine the parentage of 57 nestlings (19 broods). Polygamous mating was involved in 26% of the broods and reproduction was shared among group members of both sexes in at least three groups. Immigrants of both sexes can therefore gain access to mates by living in a group, while reproduction is unlikely to involve nondispersers. This implies that nondispersers and immigrants gain different sorts of benefits from group living and helping at the nest. Our genetic data confirmed that nondispersers associated with their parents on the natal territory and therefore that delayed natal dispersal leads to family formation in the carrion crow. Polygamous mating was not found in groups without immigrants, suggesting that, in this population, breeders lose parentage in their brood when sociality is extended beyond the limit of the nuclear family.Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.      

Assessing the cost of helping: the roles of body condition and oxidative balance in the Seychelles warbler (Acrocephalus sechellensis)

In cooperatively breeding species, helping close relatives may provide important fitness benefits. However, helping can be energetically expensive and may result in increased generation of reactive oxygen species. Consequently, an oxidant/antioxidant imbalance can lead to higher oxidative stress susceptibility. Given the potential costs of helping, it may be that only individuals with a sufficiently good body condition and/or stable oxidative balance can afford to help. Knowledge about relationships between social status and oxidative balance in cooperatively breeding systems is still limited. Studying these relationships is important for understanding the costs of helping and physiological pressures of reproduction. Here we evaluate the relationship between helping behaviour, body condition and oxidative balance in a wild population of the cooperatively breeding Seychelles warbler (Acrocephalus sechellensis). In this species, some subordinate individuals help dominant birds with the rearing of young, while others refrain from any assistance. We assessed body condition and oxidative parameters of birds of different social status caught during different breeding stages. We found that, prior to breeding, female subordinates that did not subsequently help (non-helpers) had significantly lower body condition and higher ROMs (reactive oxygen metabolites) than helpers and dominants. During the later stages of breeding, body condition was low in dominants and helpers, but high in non-helpers. Differences in oxidative balance between individuals of different social status were found only during nest care: Dominant males occupied with guarding behaviours tended to have relatively high oxidative stress susceptibility. Furthermore, dominant and helper females showed elevated antioxidant capacity (measured as OXY) in the weeks just prior to egg-laying, possibly representing a change in their reproductive physiology. The results imply that an individuals' oxidative balance may be influenced by factors related to reproduction, which can differ with sex and--within cooperative breeding systems--social status.     

Cooperative breeders adjust offspring sex ratios to produce helpful helpers

Whether birds and mammals adaptively adjust their offspring sex ratios in response to their environment is much debated. A source of confusion is that different studies show different patterns, with sex ratio adjustment appearing to occur in some cases but not others. The extent to which this reflects interesting biological variation due to differences in the underlying selective forces, as opposed to statistical noise, is not clear. Cooperatively breeding species offer an opportunity to address this problem because the strength of selection on sex ratio adjustment can be estimated. When helping behavior is sex dependent, parents are predicted to overproduce the helping sex when this sex is rare or absent. We show here that the extent of this behavior depends on the benefit that helpers bring to parents: there is greater sex ratio adjustment when helpers bring larger benefits. Variable selection on sex ratio adjustment may thus explain variable empirical findings.