There is enough research on it, but it often looks at the problem through the lens of incest rather than whole community health and trajectory. E.g. for the UK Biobank sample [1]:
> We thus identified 125 unrelated participants (65 males and 60 females) whose genomes are consistent with their parents being first- or second-degree relatives. That represents a prevalence of [extreme inbreeding] ~0.03%, i.e., ~1/3652
> It is worth noting that complex inbreeding loops between second degree-relatives may also lead to extreme values of FROH. However, mating between first-degree relatives remains a more parsimonious explanation of the empirical observations, in particular in a population of European ancestry where such complex inbreeding loops are uncommon
> We quantified the consequences of EI on multiple traits measured in the UKB. We first analysed ten control traits with prior evidence of inbreeding depression4,8,10,13. Those ten traits are height, hip-to-waist ratio (HWR), handgrip strength (HGS; average of left and right hand), lung function measured as the peak expiratory flow (PEF), visual acuity (VA), auditory acuity (AA), number of years of education (EA), fluid intelligence score (FIS), cognitive function measured as the mean time to correctly identify matches (MTCIM) and fertility measured as the number of children (NCh). ...
> As expected, we found that EI cases had a reduced mean in these ten traits as compared to EI controls. More specifically, we found phenotypic means in EI cases to be between 0.3 and 0.7 standard deviation below the population mean (Table 3). Note, that under normality assumptions, between ~25 and ~40% of the population has a phenotype below 0.7 and 0.3 standard deviations below the mean, respectively.
> We also specifically estimated the inbreeding load (often denoted B), which represents the number of loci with deleterious alleles that would cause one death on average if made homozygous3 ... and found an estimate of B ~1.46
And for India [2]:
> A cohort of 408 children (6 to 15 years of age) was selected from inbred and non-inbred families of five Muslim populations of Jammu region.
> We found significant decline in child cognitive abilities due to inbreeding and high frequency of mental retardation among offspring from inbred families. The mean differences (95% C.I.) were reported for the VIQ, being −22.00 (−24.82, −19.17), PIQ −26.92 (−29.96, −23.87) and FSIQ −24.47 (−27.35, −21.59) for inbred as compared to non-inbred children (p>0.001). The higher risk of being mentally retarded was found to be more obvious among inbred categories corresponding to the degree of inbreeding and the same accounts least for non-inbred children (p<0.0001).
The above reasoning explains the near-universal human taboo against this guaranteed way to increase occurrence of homozygosity and run into inbreeding depression – instantly or in a small number of generations. But small isolated traditional communities risk turning effectively incestuous, and some traditions even normalize cousin marriages. E.g. in Pakistan it's a big problem and is researched intensely [3]:
> Tribal and caste systems are deeply rooted in remote areas of Pakistan. Cheema said that the caste system, particularly among the Arain people living in Punjab province, is especially rigid and leads to many inter-family marriages. She said several genetic disorders are commonly found in this community.
Effective population sizes in those endogamous clan societies are orders of magnitude lower than census sizes, they can be in the low dozens for a given group.
If we return to pedigree dogs (in UK again), the overall situation is admittedly not great but there's cause for optimism. Ironically, one of the most stereotypically "fancy" and purebred dog breeds is in the best position: [4]
> For all breeds, the trend was for the rate of inbreeding to be highest in the 1980s and 1990s, representing a major contraction in genetic diversity. Since 2000 however, the general trend has been for the rate of inbreeding to decline to sustainable levels, with some modest restoration of genetic diversity in some cases.
> Rate of inbreeding (ΔF) and effective population size (Ne) Of all breeds with an average of >50 registrations per annum (over each of the seven 5-year blocks, n = 121), five had a negative whole period ΔF implying an apparent overall increase in genetic diversity, and consequently no determinable Ne (Bernese Mountain Dog, Briard, Standard Poodle, Rhodesian Ridgeback and Tibetan Terrier). Of the 116 remaining breeds, the Ne calculated over the period 1980–2014 ranged from 23.8 (Manchester Terrier) to 918.8 (Borzoi). Of these 116 breeds, 68 had Ne of <100, with 29 having Ne of <50.
In wildlife conservation, Ne≥50 is the minimum for short-term population viability (animals are also smart enough to generally avoid mating with close relatives), but isn't considered good by any means. Still, if we adopt some utterly trivial mitigations like not using the same sire to produce 20% of the next generation of pedigreed dogs, we'll quickly eliminate the bulk of the problem.
> We thus identified 125 unrelated participants (65 males and 60 females) whose genomes are consistent with their parents being first- or second-degree relatives. That represents a prevalence of [extreme inbreeding] ~0.03%, i.e., ~1/3652
> It is worth noting that complex inbreeding loops between second degree-relatives may also lead to extreme values of FROH. However, mating between first-degree relatives remains a more parsimonious explanation of the empirical observations, in particular in a population of European ancestry where such complex inbreeding loops are uncommon > We quantified the consequences of EI on multiple traits measured in the UKB. We first analysed ten control traits with prior evidence of inbreeding depression4,8,10,13. Those ten traits are height, hip-to-waist ratio (HWR), handgrip strength (HGS; average of left and right hand), lung function measured as the peak expiratory flow (PEF), visual acuity (VA), auditory acuity (AA), number of years of education (EA), fluid intelligence score (FIS), cognitive function measured as the mean time to correctly identify matches (MTCIM) and fertility measured as the number of children (NCh). ...
> As expected, we found that EI cases had a reduced mean in these ten traits as compared to EI controls. More specifically, we found phenotypic means in EI cases to be between 0.3 and 0.7 standard deviation below the population mean (Table 3). Note, that under normality assumptions, between ~25 and ~40% of the population has a phenotype below 0.7 and 0.3 standard deviations below the mean, respectively.
> We also specifically estimated the inbreeding load (often denoted B), which represents the number of loci with deleterious alleles that would cause one death on average if made homozygous3 ... and found an estimate of B ~1.46
And for India [2]:
> A cohort of 408 children (6 to 15 years of age) was selected from inbred and non-inbred families of five Muslim populations of Jammu region.
> We found significant decline in child cognitive abilities due to inbreeding and high frequency of mental retardation among offspring from inbred families. The mean differences (95% C.I.) were reported for the VIQ, being −22.00 (−24.82, −19.17), PIQ −26.92 (−29.96, −23.87) and FSIQ −24.47 (−27.35, −21.59) for inbred as compared to non-inbred children (p>0.001). The higher risk of being mentally retarded was found to be more obvious among inbred categories corresponding to the degree of inbreeding and the same accounts least for non-inbred children (p<0.0001).
The above reasoning explains the near-universal human taboo against this guaranteed way to increase occurrence of homozygosity and run into inbreeding depression – instantly or in a small number of generations. But small isolated traditional communities risk turning effectively incestuous, and some traditions even normalize cousin marriages. E.g. in Pakistan it's a big problem and is researched intensely [3]:
> Tribal and caste systems are deeply rooted in remote areas of Pakistan. Cheema said that the caste system, particularly among the Arain people living in Punjab province, is especially rigid and leads to many inter-family marriages. She said several genetic disorders are commonly found in this community.
Effective population sizes in those endogamous clan societies are orders of magnitude lower than census sizes, they can be in the low dozens for a given group.
If we return to pedigree dogs (in UK again), the overall situation is admittedly not great but there's cause for optimism. Ironically, one of the most stereotypically "fancy" and purebred dog breeds is in the best position: [4]
> For all breeds, the trend was for the rate of inbreeding to be highest in the 1980s and 1990s, representing a major contraction in genetic diversity. Since 2000 however, the general trend has been for the rate of inbreeding to decline to sustainable levels, with some modest restoration of genetic diversity in some cases.
> Rate of inbreeding (ΔF) and effective population size (Ne) Of all breeds with an average of >50 registrations per annum (over each of the seven 5-year blocks, n = 121), five had a negative whole period ΔF implying an apparent overall increase in genetic diversity, and consequently no determinable Ne (Bernese Mountain Dog, Briard, Standard Poodle, Rhodesian Ridgeback and Tibetan Terrier). Of the 116 remaining breeds, the Ne calculated over the period 1980–2014 ranged from 23.8 (Manchester Terrier) to 918.8 (Borzoi). Of these 116 breeds, 68 had Ne of <100, with 29 having Ne of <50.
In wildlife conservation, Ne≥50 is the minimum for short-term population viability (animals are also smart enough to generally avoid mating with close relatives), but isn't considered good by any means. Still, if we adopt some utterly trivial mitigations like not using the same sire to produce 20% of the next generation of pedigreed dogs, we'll quickly eliminate the bulk of the problem.
1. https://www.nature.com/articles/s41467-019-11724-6
2. https://journals.plos.org/plosone/article?id=10.1371/journal... –
3. https://www.dw.com/en/pakistan-cousin-marriages-create-high-...
4. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4579366/