It is a very interesting question to make. After all, it is a country divided along ethnic lines that one could well argue that never finished its unification process, where the periphery, particularly Basques and Catalans, often feel opressed by an ethnic Castilian/Spanish rule.
But in theory all Spanish citizens are equal. And that has been largely the case since 1715, when the new Bourbon monarch supressed the self-rule of the realms of the Crown of Aragon, after the War of Spanish Succession. While it is often argued by Spanish historians that Spain was founded in the late 15th century, the reality is that it was only formed as a unitarian state (with the Basque exception) with this first Bourbon monarch, Philp V, more than 200 years later, after the other European posessions of the crown were lost (mostly to Austria).
I have been checking the origins (place of birth and/or where the family was estabilished) of all Prime Ministers (and the few non-monarchical chiefs of state) of Spain since this Philip V, founder of Spain as we know it (and also first to estabilish the charge of Secretary of State, that would later become Prime Minister - or "President of the Goverment", as the office is nowadays officially known in Spain). Skipping the interim ones and those who ruled less for than a full month (most of which are hard to find out about their origins, as they are widely unknown) , they make a total of 115, of which 95 can be safely known their origins and these were within modern Spain.
But before jumping to conclussions, let's define (admittedly with some arbitrariety) which are the ethnicities of Spain. I avoided to use the 15 (and two colonial bits of Morocco) modern Autonomous communities as such. It could have been a criteria but really many of those communities are just but capricious subdivisions of Castile with little or no distinct personality. Instead, I followed a more historical approach:
Above you can see the Iberian peninsula (then known as Spain) as it used to be in the 13th, 14th and 15th centuries. Then (and later on with the Habsburg dynasty) Castile was a unitarian kingdom, divided in provinces (in some cases called "kingdoms" because of historical reasons but with no autonomy) but Aragon was instead a federative crown formed (in Iberia) of four realms: Aragon, Catalonia, Valencia and Mallorca (Balearic Islands). Additionally (we will ignore Portugal here) there was Navarre and the three western Basque provinces that, even if annexed to Castile, were almost totally autonomous.
The Spanish constitution of 1978 recognizes four "historical nationalities": Basque Country, Catalonia, Galicia and Andalusia. The first two had autonomy historically but the latter two did not: Galicia is the "Portuguese speaking" (ok, ok, "Galician-speaking" but it's the same) part of the kingdom of Castile and, well, Andalusia... they have a strong accent and once were Muslims. In fact there was some controversy on wether Andalusia was a "nationality" but I will admit that they have their peculairities. So excepting Galicia and Andalusia, as well as the Basque Country, I will consider all the historical Kingdom of Castile (or Castile and Leon) as a single ethnical unity: Castile. I will also consider apart the Canary Islands because they are, in my opinion, more of a historical nationality than Andalusia and, as colonial posession, they definitively deserve a place apart.
I will also consider separate ethnicities here the four realms of the crown of Aragon (now each a separate autonomous community) and the Basque Country (Navarre included).
Comparing with the official administrative division active for the last few decades, then I will consider the following ethnicities:
- Galician: Galicia
- Andalusian: Andalusia
- Catalan: Catalonia
- Aragonese: Aragon
- Valencian: Valencian Community
- Balear: Balearic Islands
- Basque: A.C. of the Basque Country and F.C. of Navarre
- Castilian: Asturias, Cantabria, Castile-Leon, La Rioja, Madrid, Extremadura, Castile-La Mancha and Murcia
I know it's arguable but it's also quite reasonable.
So where are the 115 historical Spanish prime ministers from? Voilá:
- 47 Castilian
- 26 Andalusian
- 7 Galicians
- 4 Valencians
- 3 Basque
- 3 Catalans
- 2 Aragonese
- 2 Balearic
- 1 Canarian
- 6 from other European origins (mostly in the 18th century: 2 Italians, 2 French, 1 English and 1 Irish)
- 6 from former colonies (mostly in the 19th century: 2 from Argentina, 2 from Cuba, 1 from Mexico and 1 from the Philippines)
- 8 I could not locate their origins with certainty
So there are 20 PMs that are useless for our purpose (foreigners, creole Spaniards or of unknown origin). But we can still consider the remaining 95.
The first element that strikes is that 73 prime ministers (76.8% of all) are either Castilian or Andalusian, while only three are Catalans (and two of them were presidents of the First Republic). There are only three Basques too but Basques are quite less in number than Catalans. Without even getting into figures yet, I know that Catalonia can be compared with Andalusia by population, and the Basque Country with Galicia. And it is quite striking that the Andalusian PMs are almost nine times the Catalan ones.
But let's compare data: population vs. number of Prime Ministers. We can get a discrimination ratio (positive or negative) from those figures.
- Castile (as defined above) has 14.9 million people, 33.5% of the Spanish total, but it has 49.5% of the historical rulers. It has an excess (possitive discrimination ratio) of +48%.
- Andalusia has 7.84 million inhabitants, 17.9% of Spanish citizens... but it had 27.4% of the prime ministers. It has a discrimination ratio of +53%.
- Catalonia has 7.00 million people, 15.9% of Spain's total but only 3.16% of the historical rulers. Its discrimination ratio is -80%. Would it not have been for the failed First Republic, it would have been much worse.
- Valencia has 4.69 million people, 10.6% of the total but only 4.21% of historical PMs. Its discrimination ratio is -60%.
- Galicia has 2.76 million people, 6.3% of the total and 7.37% of historical rulers. Its discrimination ratio is +17%.
- The Southern Basque Country has 2.72 million people, 6.2% of Spain, but only 3.16% of the historical rulers. Its discrimination ratio is of -49%.
- The Canary Islands have 1.97 million people, 4.5% of Spain, but only 1.05% of the historical rulers (one). Their discrimination ratio is almost as bad as that of Catalonia: -77%.
- Aragon has 1.27 million people, 2.9% of Spaniards, and had 2.11% of rulers. Its discrimination ratio is -28%.
- The Balearic Islands have 980,000 inhabitants, 2.2% of Spanish citizens, and had 2.11% of PMs. Its discrimination ratio is nearly null (in spite of being Catalan speakers): -4%.
From these figures we can see that there are the following categories:
1. Most favored ethnicities: Andalusians and Castilians
2. Almost neutrally treated ethnicities: Galicians, Balears and Aragonese
3. Clearly discriminated ethnicities: Catalans, Canarians, Valencians and Basques
Maybe Catalans and Basques have the biggest share of economic power (as from the gross internal product of these countries) but when it comes to political power, they are among the most discriminated against ethnicities of Spain, together with Valencians (largely Catalan-speakers) and Canarians (the first and last overseas colony). Castilians (Andalusians included) rule the country clearly.
Raw data and maps from Wikipedia (English and Spanish language versions).